The Rarest Conditions Known To Science

The human body is a complex system that is far from perfect. Things can go wrong with it all the time, but it’s not always bad news. Sometimes, it just means we might stand out a little more than everyone around us.

Plenty of the most unusual conditions are actually pretty rare, so there aren't always a lot of documented instances. This means it's harder for doctors and scientists to come up with a cure, assuming one can even truly exist. In fact, some of the rarest conditions known to science still have people scratching their heads.

Methemoglobinemia is one of the rarest conditions that can turn someone's skin blue 

• Symptoms: Nausea, shortness of breath, seizures, blue skin
• Rarity: Estimated 3,000 people a year

If you were asked about people with blue skin, you might think someone was referring to the Smurfs. However, believe it or not, there are some people in the world whose skin is this color thanks to a condition known as methemoglobinemia. This is when a person's body produces an excess of methemoglobin, a form of hemoglobin.

Someone with one of these rare conditions can suffer symptoms like nausea, shortness of breath, and possibly even seizures, along with this cyanosis - the blue skin. While methemoglobinemia can be genetic, it's more typically acquired through medications like antibiotics. Fortunately, oxygen therapy and other treatments have a relatively good success rate when it comes to resolving the condition.

Polydactyly can give a person extra fingers and thumbs 

• Symptoms: Extra fingers or toes
• Rarity: Estimated 1 in every 3,000 people

If you've ever thought that five fingers aren't enough for one hand, you might be surprised to learn that there are people out there with more. This is due to one of the world's rarest conditions, known as polydactyly. People with this condition can develop extra fingers on their hands or toes on their feet to varying degrees.

Someone with polydactyly could have a fully-functioning sixth finger that doesn't actually look out of place. However, another person with this condition could have an extra finger that's more like a small growth than another digit. Treatments can vary depending on the type of polydactyly someone has, but surgery is usually an option to resolve any aesthetic concerns.

Uncombable hair syndrome makes it impossible to keep a person's hair flat 

• Symptoms: Dry hair, frizzy hair, silvery hair, hair that doesn't lay flat
• Rarity: At least 100 documented cases

Most people are familiar with having bedhead, but at least it's just a temporary thing, right? After a good shower or five minutes with a hairbrush, the problem is generally resolved. Unfortunately, it is for most people, but not for those with one of these unique conditions. Uncombable hair syndrome is a real problem that affects some people.

As the name suggests, someone with this condition can never comb their hair to be flat. It usually develops in children up to 12 years old and is typically characterized by dry, frizzy, silvery hair. There is no definitive way to treat uncombable hair syndrome - which apparently affects around 100 people - but it does tend to regress as time progresses.

Black hairy tongue is an unusual but benign condition affecting someone's mouth 

• Symptoms: Discolored bumps on tongue, black marks on tongue
• Rarity: Estimated 0.5% of people

Have you ever eaten something colorful and then looked at your tongue afterward? It's the type of thing many children do after eating certain candy. Fortunately, this color change only lasts minutes. The same can't be said for those with black hairy tongues, one of the rarest conditions in the world. Thankfully, it's also one of the more harmless conditions.

Someone with this issue has elongated bumps on their tongue, which are discolored, hence why their tongue looks black and hairy. It can result from things like certain medications, poor oral hygiene, or a diet of soft foods. It reportedly affects 0.5% of adults and can gradually be resolved through improving oral hygiene and regularly brushing the tongue.

Vitiligo is another of the rarest conditions that can affect a person's skin color 

• Symptoms: White patches on skin
• Rarity: Estimated 0.5% - 1% of people

If you've ever noticed someone with seemingly random white patches on their skin, they might have vitiligo. This is where the skin lacks melanin, robbing it of the color that should be on the body. It's most noticeable in those with darker complexions, with the model Winnie Harlow being one of the most famous spokespeople for this unusual condition.

It's unknown exactly why vitiligo happens, although studies indicate it might have something to do with changes to a person's immune system. There are ways someone with vitiligo can treat their condition, such as using phototherapy or applying certain medicated creams. However, it's not a life-threatening condition, so these treatments are only really necessary for aesthetic reasons.

Gigantism brings a person more problems than jokes about the weather

• Symptoms: Headaches, insomnia, deafness, heart issues, delayed puberty
• Rarity: Estimated 100 documented cases

A person's height can sometimes be quite a sore spot for someone. If someone is pretty small or tall, they can get mocked for it, particularly when they're younger. To make matters worse, it can also lead to health problems if their height is tied to an unusual condition like gigantism. Rumeysa Golgi is a good example of this.

She's more than seven feet tall and was named the world's tallest woman in October 2021. Unfortunately, being so tall has left her with mobility issues because of muscle weakness. Other issues that can crop up with this condition include recurring headaches, insomnia, delayed puberty, deafness, and heart issues. Jokes about the weather are generally the least of their concerns.

People with pica often have cravings for things that aren't food 

• Symptoms: Cravings for nonfood items
• Rarity: Estimated 1.3% - 8% of people

Eating disorders are, unfortunately, not one of the rarest conditions in the world. However, some of these are more common than others. While certain conditions have become increasingly talked about, something like pica is still an often unfamiliar problem to many people. This is where someone struggles with cravings to eat things that aren't actually food.

Pica can be quite a serious condition, given that what people eat can be dangerous for their health. For instance, there have been reports in the past of children with pica eating painted plaster containing lead, which has resulted in brain damage. How to treat this condition varies, but psychological approaches are usually the first port of call.

Cotard's syndrome leads someone to believe that they're no longer alive 

• Symptoms: Feeling anxious, lack of social skills, feeling disconnected from yourself, starvation
• Rarity: Estimated 200 cases worldwide

Many people fear the end of their life. Unfortunately for some, that nightmare becomes reality while they're still alive. We're referring to people living with Cotard's syndrome, a disorder when someone believes they're no longer alive, they don't exist, or something else along those lines. It's apparently due to a malfunction in the fusiform gyrus section of the brain.

Haley Smith was someone who apparently lived with Cotard's syndrome for three years, although working with a therapist reportedly helped her to gradually recover from this disease. The teenager was lucky, given that this unusual condition can often lead to people passing away for real. This is often due to starvation, with sufferers assuming they no longer need to eat. 

Someone with foreign accent syndrome might suddenly sound different than usual 

• Symptoms: Speaking with a different accent, using a different pitch to speak
• Rarity: Estimated 100 people since 1907

If someone suddenly started talking to you in a different accent, you'd probably be caught off guard. However, there have been instances of this happening, albeit not in the middle of a conversation. It's known as foreign accent syndrome and is one of the rarest conditions in the world. It's most commonly the consequence of a stroke or head trauma.

When this happens, it might sound like someone is suddenly talking with something like a northern British accent or a southern French accent. Instead, it's simply damage to parts of the person's brain causing them to mispronounce syllables, speak in an altered pitch, and so on. While this is one of the more unusual conditions, it's generally a temporary issue.

Parasitic twins can occur when a twin embryo fails to fully separate during pregnancy 

• Symptoms: Extra motionless limbs when born
• Rarity: Estimated 1 in every 1 million births

Although twins are usually a blessing, having multiple kids growing inside one body can increase the chance of birth complications. This includes the risk of a parasitic twin, which is when the twin embryo doesn't fully separate during development. As a result, one of the embryos becomes dominant while the other stops developing and becomes reliant on their twin.

How a set of twins where one is parasitic looks can vary significantly from person to person. Often, though, the dominant twin will be born with certain body parts of their twin attached to them. With fewer than one in 1 million births reportedly producing a parasitic twin, it's one of the world's rarest conditions.

Fimbriated folds look like small tentacles underneath a person's tongue 

• Symptoms: Elongated flesh under tongue
• Rarity: Unknown

Most people don't pay that much attention to what's going on underneath someone's tongue. However, this can sometimes be something worth looking out for, especially if a person has fimbriated folds. These look like small tentacles which may appear concerning but are otherwise harmless. Still, they can sometimes be a nuisance if they get stuck in someone's teeth.

It's unclear precisely how this ranks out of all the rarest conditions in the world. However, given that many aren't aware that fimbriated folds even exist, it's evidently not something that affects most people. Luckily, because of their placement, someone who does have these on their tongue doesn't have to worry about it being spotted all the time.

A water allergy is an unusual condition that can be a daily struggle 

• Symptoms: Itchy skin, swollen skin, shortness of breath, wheezing
• Rarity: Estimated 1 in every 230 million people

Allergies, as a whole, aren't that rare. Many people have something their body doesn't respond well to, although some definitely suffer more than others. However, when we break down all the allergies, one of the rarest is being allergic to water. Given how much this stuff forms a daily part of someone's life, it's not a pleasant condition to have.

A water allergy - or aquagenic urticaria - often results in someone's skin becoming itchy, swelling up, or being covered in hives when exposed to water. In some cases, symptoms like wheezing and shortness of breath are also possible, but this is generally just in rare cases. Although there is no cure for this unusual condition, applying antihistamines before exposure can help.

Albinism is a lifelong skin condition that can pose some problems 

• Symptoms: Pale skin, pale hair, white eyelashes and eyebrows, issues with eyesight
• Rarity: Estimated 1 in every 5,000 - 15,000 people

If vitiligo is what happens when parts of a person's skin lacks melanin, albinism is what happens when someone's entire body lacks this pigment. In terms of humans, this is another of the rarest conditions in the world, although there are some animals where it's a bit more common. It can apparently even affect some plants, too.

A person with albinism will likely have pale skin and hair - including eyebrows and eyelashes - as well as potential problems with their eyesight. Someone with albinism won't necessarily live a shorter lifespan, nor should their condition worsen over time. However, a person with this unique condition may also struggle with exposure to the sun due to the lack of melanin.

Congenital insensitivity to pain is more dangerous than it might initially seem 

• Symptoms: Frequent injuries, numbness to pain
• Rarity: Estimated 1 in every 25,000 people

The notion of not being able to perceive physical pain might seem like a blessing sometimes, especially when you're in agony. However, that doesn't mean those with congenital insensitivity to pain are lucky - far from it. Someone born with this unusual condition will often end up hurting themselves more because their body doesn't tell them when they're in danger.

For instance, it's not uncommon for people with this condition to burn themselves because they don't realize what the fire is doing to their bodies. Sadly, it's apparently quite common for people with congenital insensitivity to pain to actually lose their lives early. That's heartbreaking for the roughly one in every 25,000 people that have this condition.

Dermatographism is a condition where scratches on the skin last longer than usual 

• Symptoms: Redness on skin, inflammation, itchiness, swelling
• Rarity: Estimated 2% - 5% of people

How often have you needed to write something down but you don't have anything suitable to hand? For someone with dermatographism, that usually isn't a problem. After all, a person with this condition can scratch something on their skin, and it typically sticks around for around 15-30 minutes. It's an allergic-like reaction, but it's not actually an allergic reaction.

The symptoms of dermatographism are believed to be the result of histamine being released on the surface of the skin. Therefore, taking antihistamines is usually considered one of the best ways to prevent the reaction from happening. Avoiding hot baths and showers can apparently also be quite beneficial, as the heat can reportedly influence symptoms to occur.

Hutchinson-Gilford progeria syndrome is a dangerous illness that affects a child's development 

• Symptoms: Low weight, growth delays, premature aging, lack of hair, prominent eyes, a larger head
• Rarity: Estimated 1 in every 4 million births

It's an unfortunate fact that some of the rarest conditions in the world are also some of the most dangerous. Hutchinson-Gilford progeria syndrome is sadly one of these. This is a condition that apparently affects one in every 4 million newborns worldwide, with less than a few hundred cases reported in scientific literature since the late 1800s.

This unusual condition is characterized by premature aging, with growth delays and low weight also expected. Someone with Hutchinson-Gilford progeria syndrome will often have a face that's disproportionate to their head, prominent eyes, and a lack of hair. Unfortunately, the average life expectancy with this condition is just 13 years, although some have made it to their 20s.

Stiff-person syndrome can cause extreme rigidity and sensitivity to certain senses

• Symptoms: Muscle spasms, stiffness in body, sensitivity to some senses
• Rarity: Estimated 1 in every 1 million people

Most people know what it's like to have stiff limbs every now and again, especially after a workout. However, this sort of discomfort after exercise is nothing like what those with stiff-person syndrome have to deal with. One of the world's rarest conditions, it's a neurological disorder characterized by progressive rigidity and stiffness in the body.

This unusual condition apparently affects one in every million people and is usually found in those who are middle-aged. As the disorder worsens, it can lead to spasms and sensitivity to various senses, along with a significant impact on someone's mental wellbeing. Unfortunately, there's no clear way to treat this condition outside of helping to make a person more comfortable.

Ochoa syndrome affects both a person's smile and how they process body waste 

• Symptoms: Unusual facial expressions, lower digestive issues, kidney failure
• Rarity: Estimated 150 reported cases

Has anyone ever criticized you for the way that you smile? It's something that a surprising amount of people actually get quite self-conscious about. What's more, some can't even help the way that they smile because they have Ochoa syndrome. This is a congenital disorder where someone appears to have an inverted facial expression whenever they smile.

This unusual condition doesn't just affect someone's face either. It's also reportedly characterized by lower urinary tract and bowel dysfunction, which can cause issues like renal failure if not properly handled. Antibiotics can help to avoid infection, with catheterization also reportedly a possibility. What's more, those inverted facial expressions can actually be beneficial to diagnosing someone early.

Eagle syndrome can cause sharp pain around a person's jaw due to an elongated styloid process 

• Symptoms: Sharp pain in the jaw, tongue, and throat
• Rarity: Estimated 0.16% of people

One of the world's rarest conditions is eagle syndrome, which is most frequently associated with sharp pain in the upper parts of the body. These include the jaw bone, the base of the tongue, and the back of the throat. This is reportedly due to a misshapen or elongated styloid process, which is a small bone beneath the ear.

Calcification of the stylohyoid ligament can also cause eagle syndrome, which can typically be dealt with via surgery. The success rate for this is reportedly 80%, with medications also sometimes necessary to ensure a full recovery. Studies have found that around 0.16% of people reportedly have eagle syndrome, although more have an elongated styloid process without these issues.

Coloboma is an eye condition that can differ significantly regarding whether it's a problem 

• Symptoms: A keyhole-shaped pupil
• Rarity: Estimated fewer than 1 in every 10,000 people

The eyes are often referred to as the window to someone's soul. They can also be a good indicator of whether or not someone has one of the world's rarest conditions. There are a few eye conditions that aren't seen that often, including coloboma. This is when part of the tissue that forms someone's eye is missing.

You can typically notice when someone has coloboma because their pupil will look like it's seeping into their iris, typically in a keyhole shape. Less than one in 10,000 births reportedly develop coloboma, with the severity of this condition ranging quite significantly. Some who have it suffer no vision problems, while others can only see light or dark.

Dropped head syndrome forces people to constantly look at the floor 

• Symptoms: A hunched neck and shoulders, headaches, pain in the neck, trouble balancing
• Rarity: Estimated 1.3% of people

Maintaining good posture is ideal if you want to avoid developing problems later in life. Unfortunately, some people have unusual conditions that make this impossible, such as dropped head syndrome. This is characterized by a difficulty with keeping the head up, prompting sufferers to look like they're hunched over and constantly staring down at their feet.

Other issues that can emerge from this include trouble balancing, pain in the head and neck, a lack of cervical range of motion, and a general decline in a person's quality of life. There's no one cause behind dropped head syndrome, but a surgical procedure can often resolve the issues that have developed with the neck and spine.

Ulnar dimelia often looks like two hands have fused into one 

• Symptoms: Duplicated number of bones and fingers in hand
• Rarity: Less than 100 documented cases

One of these unusual conditions is a rare one indeed. Less than 100 cases have reportedly been recorded in medical literature, so experts don't know as much about it as they do many other conditions. We're referring to ulnar dimelia, something also known as mirror hand, when the duplication of various bones gives someone's hand this peculiar appearance.

Ulnar dimelia won't always look the same for everyone. However, generally speaking, it typically looks like two hands that have fused into one. Some feel that this results in the hand bearing some resemblance to an arachnid. Treatment for this unique condition may entail surgical amputation to reduce the number of fingers on one hand.

Persistent pupillary membranes are membranes that don't disappear when someone's born 

• Symptoms: Single or several strands across the pupil
• Rarity: Estimated 20% of adults

One of the rarest conditions that can affect a person's eye is a persistent pupillary membrane. This is where the remnants of a fetal membrane remain as strands of tissue across someone's pupil. The purpose of the pupillary membrane is to provide a blood supply to the lens, but it's supposed to atrophy within weeks of being born.

Thankfully, this is a condition that's primarily aesthetic, with a persistent pupillary membrane rarely causing any other symptoms. Occasionally, if the strands connect to the lens or cornea, it can cause small cataracts or corneal opacities. However, a topical atropine can apparently help to dilate an affected pupil and sometimes ensure the breakdown of this condition.

Microcephaly is a genetic condition that stunts the growth of someone's head 

• Symptoms: Smaller than average head, learning disabilities
• Rarity: Estimated 1 in every 5,000 births

In most cases, a person's head is usually proportionate to the rest of their body. However, someone with microcephaly is different because their head doesn't develop in the same way as the average person's. Unfortunately, this means that people with this unusual condition often deal with intellectual disabilities because their brain is unable to grow as large as it should.

This is apparently a genetic condition that forms early in pregnancy, although someone with microcephaly might not display symptoms for several years. There is no cure for this condition, but there are ways to cope with specific symptoms. Regarding rarity, it's estimated by the CDC that up to one in every 5,000 US babies is born with microcephaly.

Stahl's ear gives a person pointed ears like a fantasy creature 

• Symptoms: A folded tip of the ear into a point
• Rarity: Estimated 5% - 7% of people

It's common in fantasy media for certain human-like creatures to have pointed ears. Surprisingly, this isn't something restricted solely to fiction. There are instances of people in real life also having such ears, and it's one of the rarest conditions around. The official term for it is Stahl's ear, and it's the result of an extra cartilage fold.

It's most typically seen in children, with early discovery making it possible to correct the issue without surgery. The older someone gets, the likelier it is that someone will require surgical correction. That's assuming it's what someone wants, of course. There's no indication that Stahl's ear is problematic, so it purely comes down to aesthetics.

ACDC is a condition that causes calcium build-up, and it isn't just a rock band 

• Symptoms: Pain in wrists and hands, cramp in leg muscles, pain in feet
• Rarity: Estimated fewer than 1,000 people

If we were to start talking about ACDC, you'd probably assume we were referring to the famous band. However, they aren't the only ACDC in the world. One of the rarest conditions out there also goes by this name, and it's nowhere near as fun. Its full name is arterial calcification due to deficiency of CD73.

This basically means that people develop a build-up of calcium in areas like their hands and feet, which can cause progressive pain. It's believed that fewer than 1,000 people in the United States have this disease, which generally doesn't develop in someone until they reach adulthood. It's reportedly caused by genetic changes in the NT5E gene.

Alkaptonuria causes a build-up of homogentisic acid that can cause various problems 

• Symptoms: Kidney stones, lower back pain, pain in joints, brown or gray spots in eyes, thickening of ear cartilage
• Rarity: Estimated 1 in every 250,000 people

When you use the bathroom and notice that your body waste isn't the color it usually is, that's when you know there's likely a problem. This can be an indicator that someone has alkaptonuria, a genetic disease where someone's body accumulates too much homogentisic acid. This is due to their inability to break down the amino acids tyrosine and phenylalanine.

The build-up of this acid can be dangerous, resulting in damage to cartilage and heart valves, as well as prompting kidney stones. Research is reportedly ongoing to try and deal with this unusual condition, which apparently affects one in 250,000 people. However, certain medications can apparently relieve pain and suppress acid production, while joint replacements can fix cartilage damage.

Kuru is a dangerous illness that was spread through the consumption of other people 

• Symptoms: Pain in arms and legs, coordination issues, headaches, difficulty walking, tremors
• Rarity: Estimated 1 in every 1 million people

When it comes to the rarest conditions in the world, kuru is definitely one that you don't want. It's incurable and dangerous, although the way it's most famously been transmitted isn't something you're likely to end up doing. The unusual disorder used to be common among the Fore people of Papua New Guinea, who would eat their family members.

It was apparently tradition to consume those who had passed away as a means to free their spirit. Unfortunately, this included eating people's brains, which is where they were most likely to pick up the illness. Thankfully, the Fore people have now ceased with this tradition, and no one has reportedly succumbed to it recently.

Porphyria has all sorts of symptoms, including chest pain and skin blistering 

• Symptoms: Pain in abdominals and chest, vomiting, fever, accelerated heart rate, blistering of skin in sunlight
• Rarity: Estimated 1 - 10 in every 50,000 people

Most of your internal organs are essential, so ensuring they're healthy is pretty important. Unfortunately, rare diseases can happen that make this difficult. Porphyria is one of them, which is a disorder that develops in the liver and affects other parts of the body. Abdominal and chest pain, vomiting, fever, and an accelerated heart rate are some of the symptoms.

Other symptoms can include blistering of the skin when exposed to sunlight, along with a change of color in body waste. Symptoms aren't always persistent, usually occurring as attacks that can potentially last for weeks. It's unclear exactly where this places amongst the other rare conditions in the world. However, roughly one to 100 per 50,000 people have it.

Distichiasis effectively gives a person an extra set of eyelashes 

• Symptoms: Eye pain, sensitivity to light, irritation, vision loss
• Rarity: Estimated 1 in every 10,000 people

Distichiasis is one of the rarest conditions that might not seem too bad on the surface. It's effectively where someone has an extra set of eyelashes. Given how many people try to accentuate their eyes with fake eyelashes, this might seem like a good condition to have. Still, it can cause a few problems for those with it.

Issues like eye pain, irritation, sensitivity to light, and vision loss are all possible for those with distichiasis. It's usually down to where the extra set of eyelashes grows because they're usually too close to someone's eyeball. It's a condition that apparently tends to be more prevalent in dogs than in humans, but isn’t entirely uncommon in people.

Preauricular pits give people an extra hole in their ear or ears 

• Symptoms: Holes around upper ear or between ear and face
• Rarity: Estimated less than 1% of people

People are generally accustomed to having holes in their ears, especially if they get tons of piercings. However, some people have holes that aren't meant to be there. These are known as preauricular pits, and they apparently develop on the upper ear between the face and the ear rim cartilage. Supposedly, less than 1% of the general population has them.

Some people with preauricular pits only have one affected ear, while others have two. Regardless, they're typically around from birth and aren't too problematic, although they can become infected. In cases where infection develops into a regular issue, surgery can help by effectively filling in the hole. Otherwise, a person can just live their life like normal.

Fibrodysplasia ossificans progressiva essentially turns connective tissue into bone 

• Symptoms: Difficulty walking, bone fractures, bone deformities, bone pain, scoliosis
• Rarity: Estimated 1 in every 2 million people

There’s a good chance many have never heard of some of the rarest conditions because they’re just that: rare. Sadly, fibrodysplasia ossificans progressiva is another illness that can really affect someone's quality of life, as connective tissue like muscles and tendons turn into bone. This can severely disable someone because of how it restricts their movement.

Unfortunately, while surgery can remove all this extra bone growth, it doesn't help to resolve the problem. New bone simply grows back in its place, often paving the way for things to get worse rather than better. Nowadays, it reportedly only affects one in every 2 million people, with anti-inflammatory medication sometimes offering intermittent treatment to unlucky sufferers.

Wilson's disease can have a severe impact on someone's brain and liver 

• Symptoms: Yellow skin, rings in the iris, tremors, trouble speaking, vomiting
• Rarity: Estimated 1 in every 30,000 people

You can live with some of the rarest diseases as if they don't exist. Others, though, need to be treated as soon as possible before they have dangerous consequences. Wilson's disease is one of the latter, as ignoring the symptoms can eventually result in loss of life. This is a genetic disorder where the body builds up excess copper.

Wilson's disease can have a significant impact on a person's brain and liver, resulting in symptoms like vomiting, yellowish skin, tremors, trouble speaking, and brown rings in the iris. Although it can be life-threatening, dietary changes and medication can help, as can steering clear of foods that contain copper. A reported one in 30,000 people apparently develop Wilson's disease.

Macular puckers can cause vision problems that might worsen over time  

• Symptoms: Distorted or wavy vision, blurry or loss of vision
• Rarity: Estimated 2% - 20% of people

Another of these unusual conditions that can affect a person's eyes is a macular pucker. While some of these eye conditions are simply aesthetic, macular puckers can definitely have an impact on a person's vision. They can leave someone's vision wavy or distorted, although the severity of this differs. Many people with them don't often need any treatment.

Not a great deal is known about why these happen. In most cases, doctors are none the wiser to the cause, and little can be done to fix the issue. People tend to live with the changes to their vision unless the problem is more serious. In such instances, one of two surgeries will be used - vitrectomy or membranectomy.

Chronic focal encephalitis apparently causes worsening symptoms that might be irreversible 

• Symptoms: Weakness on one side of body, seizures, vision loss, loss of motor skills
• Rarity: Fewer than 1,000 documented cases

The damage that some of the rarest conditions in the world can cause isn't always reversible. With chronic focal encephalitis, this seems to be a real issue regarding brain damage. In some cases, this damage is irreversible, meaning the loss of motor skills and mental decline is something that will never recover in the children who develop it.

Other symptoms of this illness reportedly include partial vision loss, weakness on one side, and epileptic seizures. It's unclear what causes the inflammation in the brain that sets off these symptoms, although several theories have been presented. Less than 1,000 instances of chronic focal encephalitis have apparently been reported over the years, hence why it's on this list.

RPI deficiency is one of the world's rarest conditions, with only four reported cases 

• Symptoms: Issues with vision, seizures, delayed development, decline in emotional or physical reactions
• Rarity: Only 4 documented cases

Ribose-5-phosphate isomerase, or RPI, deficiency really is one of the rarest conditions in the world. There's supposedly only one illness that's had fewer diagnoses, with four patients reportedly being diagnosed with this one. It’s effectively the result of mutations in the RPI, an enzyme of the pentose phosphate pathway, that currently has no treatment or prognosis.

The few people who develop RPI deficiency typically suffer symptoms like optic atrophy, seizures, developmental delay, and a decline in physical and emotional reactions. This unique condition seems to develop early in life, with the few reported cases all being children of varying ages. Unfortunately, a lot is still unknown about the condition due to its rarity.

Anonychia congenita is a rare condition that typically leaves people without nails 

• Symptoms: Missing or partially missing fingernails and toenails
• Rarity: Unknown

If you break your nails often, you might sometimes wish that you just didn't have them at all. That is actually a real condition that some people have, and it's known as anonychia congenita. Most of the people with this unusual condition don't have any nails on their fingers or toes, although sometimes it's just a partial loss.

Fortunately, aside from their hands and feet looking a little different, a person with anonychia congenita is no different from the average person. The condition isn't known to cause any other health problems, so the people who have it can live a normal life. How many people that accounts for is unknown as the prevalence is undocumented.

Raynaud's syndrome can turn a person's hands white in the cold 

• Symptoms: Pale or white fingers in the cold, swollen hands when warm, sores on fingertips
• Rarity: Estimated 20% of people

Do you notice a difference in your hands when you get cold? It's common for the skin to turn darker, tighten, and maybe even feel a bit numb. What isn't common is for your fingers to turn white. However, that's exactly what happens to someone who has Raynaud's syndrome. Restricted blood flow due to vasospasm is apparently responsible.

Not only can an attack cause someone's digits to lose their color, but it can also cause numbness and pain too. However, these usually only last minutes, although they can go on for a few hours. Other triggers aside from cold temperatures reportedly include anxiety and stress, with toes and parts of the face also sometimes affected.

Genu recurvatum offers a person greater leg flexibility but at a cost 

• Symptoms: Pain and weakness in leg, decreased range of motion in leg
• Rarity: Estimated 1 in every 100,000 people

People who do yoga tend to be very flexible, but there are some things even a yoga pro can't achieve unless they have one of the rarest conditions. For instance, if you're able to bend your knee backward, it's likely that you have genu recurvatum, or knee hyperextension, which is a deformity in the knee joint.

While the upside of this might seem like greater flexibility, genu recurvatum can lead to knee pain and osteoarthritis down the line. Various factors can influence this unique condition, such as cerebral palsy, instability of the knee joint, and Ehlers-Danlos syndrome. In terms of congenital genu recurvatum, it apparently happens in one in every 100,000 births.

People with situs inversus have their major visceral organs in a mirrored position 

• Symptoms: Cardiac issues
• Rarity: Estimated 0.01% of people

Something that most people learn when they're a kid is what the inside of their body looks like. However, it seems that learning about where the internal organs are isn't beneficial for everyone, given that some people have situs inversus. This is a rare congenital condition where a person's major visceral organs are in mirrored positions in the body.

So, while we're taught that a person's heart leans toward the left, someone with situs inversus actually has a heart that leans toward the right. Only 0.01% of the population reportedly have this condition, with many not actually struggling from complications, despite their bodies being designed differently. However, there are some people who do deal with cardiac problems.

Piebaldism is one of the rarest conditions that relates to a lack of melanin 

• Symptoms: White patches on skin, white streaks in hair, deafness, digestive issues
• Rarity: Estimated fewer than 1 in every 20,000 people

Whenever a part of someone's body turns white, a lack of melanin is usually to blame. Piebaldism is one of the world's rarest conditions where this is the case. The absence of melanin-forming cells results in white patches, both on the skin and in the hair, with many having a light streak in their bangs.

Of course, how this presents in a person with piebaldism varies greatly, with some people having more and larger white patches than others. Although there are connections between this condition and albinism, those with piebaldism don't usually develop the visual problems that are often prevalent. Still, other issues like congenital deafness and incomplete gastrointestinal tract development can be present instead.

Klippel-Feil syndrome is when several neck bones fuse together 

• Symptoms: Shorter neck, spina bifida, cleft palate, scoliosis
• Rarity: Estimated 1 in every 40,000 - 42,000 people

One of the rarest conditions in the world is Klippel-Feil syndrome, which reportedly affects one in every 40,000 to 42,000 births. Females are reportedly more at risk than males, with the condition resulting from the abnormal fusion of several bones in the neck. People who have it tend to have a short neck with limited mobility up here.

Klippel-Feil syndrome isn't always genetic, nor is it identifiable by birth every time. Associated abnormalities with this condition include spina bifida, cleft palate, scoliosis, Duane syndrome, and Sprengel's deformity. Surgery is an option as a symptomatic treatment, with early treatment often necessary to avoid a shortened lifespan. Unfortunately, heart defects are relatively common, affecting around 30% of patients.

Waardenburg's syndrome comes in varying types that can cause bright eyes and fused fingers 

• Symptoms: Hearing loss, bright blue eyes, eyes wider apart than usual, fused fingers
• Rarity: Estimated 1 in every 42,000 people

Waardenburg syndrome has a connection to some of the other rarest conditions in the world, such as piebaldism. However, while someone with this condition might also have a white streak in their hair, Waardenburg syndrome can also be characterized by congenital hearing loss and bright blue eyes. A wider gap between the eyes can also be an indicator of this.

The prevalence of this condition is believed to be one in 42,000, although that's across all types. The features mentioned above can be signs of type 1 and type 2, which are the most common. Type 3 and type 4, which are rarer, might also be characterized by fused fingers and Hirschsprung's disease, respectively.

Carcinoid syndrome is a series of problems that result from a carcinoid tumor 

• Symptoms: Skin flushing, rapid heartbeat, difficulty breathing, facial lesions
• Rarity: Unknown

Tumors are usually enough to make people panic, but some are rarer than others. A carcinoid tumor is one of the rarest, with just 27 new cases per million being diagnosed in the US each year. About 10% of those who have this tumor will apparently develop carcinoid syndrome, which includes skin flushing that can sometimes last for hours.

Difficulty breathing, a rapid heartbeat, and facial lesions can also be symptoms of carcinoid syndrome, which are caused by the tumor secreting chemicals into the bloodstream. Carcinoid syndrome can lead to more serious issues like carcinoid heart disease, as well as the issue of the cancerous tumor. Surgery and medications are usually necessary to try and stop further problems.

Parry-Romberg syndrome causes one side of someone's face to waste away 

• Symptoms: Shrinkage and degeneration on one side of face
• Rarity: Estimated 1 in every 250,000 people

Most people's faces aren't symmetrical. However, the asymmetry in an average person's face is far less noticeable than in someone with Parry-Romberg syndrome. Someone with this unique condition has to deal with the tissue on one side of their face shrinking and degenerating. This makes it look like one-half of them is wasting away while the rest is as expected.

The reason why Parry-Romberg syndrome occurs is apparently unknown, but it reportedly affects one in every 250,000 people. Certain immunosuppressive medications can help to manage the syndrome, although there's no clear evidence yet that these work. Surgery can also help by transferring fat to the affected side to try and restore some symmetry to the face.

Constriction ring syndrome can affect hand and foot development in random babies 

• Symptoms: Swelling of extremities, mild skin indentation
• Rarity: Estimated 1 in every 15,000 births

Constriction ring syndrome is one of the world's rarest conditions where the cause is kind of unclear. As a result, it goes by a bunch of different names, including amniotic band syndrome, congenital amputation, and ADAM Complex. Effectively, what it is is a malformation related to intrauterine bands that often affects the development of someone's hands or feet.

How these look vary from person to person, with the prevalence apparently being up to one in every 15,000 births. As it's an accidental event, there's seemingly no way to prevent it. However, this does also mean that the siblings of someone with constriction ring syndrome would likely not have the same unusual condition.

Cornelia de Lange syndrome can make someone more susceptible to repeated health issues 

• Symptoms: small stature, downturned mouth, small nose, delayed development, pneumonia, heart defects
• Rarity: Estimated 1 in every 30,000 births

Cornelia de Lange syndrome is a genetic disorder that can have a wide-ranging impact on a person's development. It affects someone both physically and cognitively, with medical challenges often arising as a result of these impacts. Some of the typical features of Cornelia de Lange syndrome include small stature, a downturned mouth, a small nose, and developmental delay.

It's believed that it affects up to one in every 30,000 births and tends to be caused by spontaneous genetic mutations. While someone with this unusual condition can live a full life, they are more likely to face certain health conditions such as recurrent pneumonia and heart defects. Failing to treat these quickly could result in a shortened lifespan.

Morgellons supposedly feels like something is crawling on or emerging from the skin 

• Symptoms: Itchy skin, stinging skin, rashes, sores, pain across skin
• Rarity: Estimated over 14,000 documented cases

Some of the rarest conditions are simply hard to diagnose. For instance, Morgellons is something that apparently isn't scientifically substantiated, with more research required to understand it better. It doesn't help that it's tied to mental health, an area in which a ton of development is still required to give people the proper treatment going forward.

Morgellons is considered by some as a delusional infestation where people believe that something is crawling on, emerging from, or stinging their skin. There are physical ailments associated with this unique condition, namely sores and rashes, that can cause a lot of pain and itching. However, it's unclear exactly what prompts these and why the condition affects people mentally.

Fragile X syndrome can leave a person with mild to moderate intellectual disability 

• Symptoms: Large ears, narrow face, flexible fingers, a low IQ
• Rarity: Estimated 1 in every 7,000 males and 1 in every 11,000 females

Fragile X syndrome is another genetic disorder, this one often having quite an impact on someone's intellectual abilities. Those with this unusual condition tend to have a lower IQ, with males reportedly averaging a score of 55. Someone with fragile X syndrome may also have certain characteristics, such as large ears, a narrow face, and more flexible fingers.

As with some of the other rarest conditions in the world, fragile x syndrome isn't something that comes with a cure. However, early intervention can ensure that someone with this condition is better capable of developing a full range of skills. Certain medications might also help for things like seizures and ADHD, which can also come with this disorder.

Human chimerism is a rare condition that sees an individual have multiple sets of DNA 

• Symptoms: Different colored eyes, different skin tones, patches of different color or textured hair
• Rarity: Estimated 100 documented cases

A chimera is basically an individual with two different sets of DNA. They can happen in animals and plants, with only around 100 cases of human chimeras apparently being documented. Singer Taylor Muhl is one such case. She was supposed to be one of a set of twins, but when her mother gave birth, she was the only baby there.

It turns out that the two eggs fused together, prompting Taylor to be born with her sister's genes. This resulted in the left side of her torso being a different color than the right side, something she'd initially thought was just a birthmark. It was years before the doctors realized that she had more than one person's DNA inside her.

Congenital contractural arachnodactyly causes a variety of symptoms, including abnormally long fingers 

• Symptoms: Tall and slim build, longer than average fingers and toes, crumpled ears, short neck
• Rarity: Estimated fewer than 1 in every 10,000 people

Congenital contractural arachnodactyly can present itself in a variety of ways. Someone with this unusual condition will likely be quite tall and slender, with that length extending to their fingers and toes. Those fingers might also be permanently flexed, while their ears may appear "crumpled." Abnormally positioned feet and a short neck can also characterize this condition.

Mutations to the FBN2 gene are apparently responsible for the onset of congenital contractural arachnodactyly. Males and females are both at risk of developing it, with the chance of passing the affected gene on reported to be 50%. How this condition is treated differs depending on the symptoms a person has, with physical therapy and surgery sometimes necessary.

Fatal familial insomnia is insomnia that can end a person's life in a matter of months 

• Symptoms: Insomnia, issues with speech, trouble with coordination, dementia
• Rarity: Estimated 40 families

Insomnia is something that plenty of people have probably experienced at some point in their lives. However, while it can be unpleasant, it's not as bad as fatal familial insomnia. This is a rare genetic condition that doesn't just make it hard to sleep but can also cause speech problems, coordination trouble, dementia, and, eventually, loss of life.

Although it's genetic, it typically doesn't make itself known until middle age. Supposedly less than 40 families around the world are known to have the mutated PRNP gene that causes this condition. Tragically, there is no cure for this condition, which simply gets worse until the person passes away. Once the symptoms start, the average survival time is 18 months.

Alice in Wonderland syndrome affects a person's perception of objects, sounds, and time 

• Symptoms: Difficulties perceiving time and sound, unable to perceive position and size of objects
• Rarity: Estimated 10% - 20% of people

Alice in Wonderland syndrome might sound relatively harmless, especially if you're a fan of the fictional story and its various movie adaptations. However, in reality, it's not that pleasant. Those with this rare condition experience distortions in their perception of objects. So, they might think that something is larger, smaller, closer, or further away than it is.

Differences in the perception of sound and time are apparently also a possibility. It's not yet known why Alice in Wonderland syndrome happens, but it has been linked to migraines, head trauma, and viral encephalitis. Children are the ones who tend to experience this condition the most, and thankfully, the symptoms don't tend to last more than an hour.

Möbius syndrome prevents a person from expressing themselves through their face 

• Symptoms: Unable to close eyes, crossed eyes, difficulties swallowing, eye sensitivity, cleft palate
• Rarity: Estimated 2 - 20 in every 1 million people

Facial paralysis is something that people with möbius syndrome generally have to deal with from the moment they're born. These people are apparently unable to close their eyes or make the expressions that humans use so frequently to convey their emotions. This can sometimes hinder their ability to connect with others who aren't aware of their condition.

Although this is a neurological disorder, a person's intelligence reportedly isn't impaired if they have möbius syndrome. Furthermore, some surgical procedures can help, such as correcting the crossed eyes that are prevalent in these cases. Perhaps unsurprisingly, this is another of the world's rarest conditions, supposedly affecting just 2 to 20 people for every one million births.

Proteus disease causes an overgrowth of tissue in various parts of the body 

• Symptoms: Rough and raised skin, bumpy and grooved lesions on skin
• Rarity: Estimated 1 in every 1 million people

Proteus disease is one of the rarest conditions in the world, reportedly impacting less than one in every one million people around the world. Only several hundred cases have ever been documented in medical literature, although there could be more undiagnosed cases out there. It's only those with obvious disfigurements that tend to be diagnosed.

People with proteus disease effectively struggle with tissue overgrowth, whether that be skin, muscles, bones, or something else. It's apparently a progressive condition, so the deformities grow worse as a child gets older. The skull, the soles of the feet, and at least one limb are where this growth tends to occur, although there is a lot of variation.

Epidermodysplasia verruciformis can give a person with tree bark-like growths 

• Symptoms: Tree bark-like growths on skin
• Rarity: Unknown

Epidermodysplasia verruciformis is sometimes known by another name - treeman syndrome. One look at a person with this incredibly rare condition usually makes it pretty clear why that is. This skin disorder - which is associated with a much higher risk of skin cancer - causes the growth of macules and papules, which look a lot like tree bark.

This condition is reportedly hereditary and is connected to increased susceptibility to the human papillomaviruses. It tends to present itself within the first 20 years of someone's life and does not yet have a curative treatment. It's unclear how prevalent this issue is, but given it's so unheard of, it's definitely one of the rarest conditions in the world.

Gut fermentation syndrome causes a person to feel like they've been drinking after eating carbohydrates 

• Symptoms: Feeling dizzy or unstable, especially after eating certain food
• Rarity: Unknown

When a person spends a night drinking, they don't always wake up the next day feeling great. It's usually their own fault for consuming as much as they did, though, right? Maybe so, but there are some people who genuinely can't help feeling worse for wear. These are the ones with gut fermentation syndrome, also known as auto-brewery syndrome.

Someone with this unusual condition effectively has to deal with all the stuff that comes with drinking without actually drinking anything. That's because ingested carbohydrates in their gastrointestinal tract get fermented due to certain bacteria or fungi in their body. Antifungals and probiotics can help to deal with the effects of this condition, as can eating fewer carbohydrates.

Hypertrichosis causes excessive hair growth, sometimes over a person's entire body 

• Symptoms: Abnormal amount of hair growth across entire body, thicker and longer hair than usual
• Rarity: Fewer than 200 documented cases

Plenty of people have strong feelings about their hair growth, usually that they either have too much of it or not enough. Those with hypertrichosis fit into the former category, and they certainly have a reason to be upset. That's because this unusual condition prompts an abnormal amount of hair growth, sometimes across the entire body.

Whether hypertrichosis is congenital or acquired later differs between patients. However, all people with this condition have excessive hair growth, which is often longer than usual, too. It's definitely one of the world's rarest conditions, given that less than 200 cases of congenital hypertrichosis have been documented since the Middle Ages. Hypertrichosis acquired later in life is apparently less rare.

Cleidocranial dysplasia affects someone's bones, including giving them hunched shoulders 

• Symptoms: Hunched shoulders, flat nose, wide-set eyes, prominent forehead, abnormal teeth
• Rarity: Estimated 1 in every 1 million births

Apparently, one in every one million births will result in cleidocranial dysplasia. This highly rare disorder impacts a person's bones, including their collarbones. Regarding this, someone with this birth defect may have poorly developed collarbones or none at all. This gives a person the appearance of having hunched shoulders. Surgery can potentially help with this, along with other symptoms.

A flat nose, prominent forehead, abnormal teeth, and wide-set eyes can also characterize someone with cleidocranial dysplasia. Gaten Matarazzo is a good example of this unusual condition, as the young actor was born with it. Over the years, he's had repeated surgeries to aid with the birth defect, which evidently hasn't harmed Gaten’s ability to forge a career.

Fields disease is a relatively unknown condition that supposedly affects just three people 

• Symptoms: Issues with balance and coordination, inability to walk and talk
• Rarity: Fewer than 3 documented cases

When talking about the rarest conditions in the world, nothing currently stands out more than Fields disease. This condition is named after the Fields twins, two of the only three people in the world to supposedly have their progressive degenerative neuromuscular disorder. The sisters are wheelchair-bound and initially didn't have any problems. This reportedly changed when they turned three.

Problems with balance and coordination reportedly grew worse over time. Although they were initially able to do things like walk, talk, and ride bikes, they were in wheelchairs before they hit double digits. Given the rarity of their disorder, it's unclear exactly how long the twins have to live. Hopefully, despite their condition, their lifespan will be relatively normal.