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Alien hand syndrome - This neurological disorder makes a person feel that their limbs are detached from their body and have a life of their own.

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Parry-Romberg syndrome - This syndrome is characterized by progressively shrinking facial tissue underneath the skin. The face becomes malformed, but usually just on one side.

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Alice in Wonderland syndrome - Also known as Todd Syndrome, this illness causes hallucinations and distorts perception. Objects around the person may be distorted in size and appear to move extremely slowly.

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Persistent general arousal disorder - According to the site Heathline, people who suffer from this disorder experience sexual arousal without sexual activity or stimulation. It is more common in women and if it occurs in men, it is called Priapism.

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Polydactyly - This is the presence of extra fingers and/or toes. Although there have been cases of people being born with extra fingers or toes that contain bones, normally, the appendages are only abnormal tissue growth.

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Hypertrichosis - This disorder is characterized by excessive hair growth.

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Cronkhite–Canada syndrome - This illness causes rare symptoms, such as chronic diarrhea, polyps in the intestines, skin pigmentation, and problems with nail growth.

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Pica - People with this disorder eat things that aren't food, such as soap or dirt.

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Fatal hilarity - This term was first coined in 1956 and is an incredibly rare way to die. A person dies from choking or having a heart attack while laughing.

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Hailey-Hailey disease - This genetic disease causes the formation of blisters and erosions on the skin. It mainly affects the armpits, neck, and folds of skin.

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Tourette syndrome - This disorder causes uncontrollable and unexpected movements or vocalizations, called tics. The first recorded case was in a small Canadian community of lumberjacks in the 19th century.

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Electromagnetic hypersensitivity - This is a sort of "invisible allergy," in which a person is extremely sensitive to the energy field around them. Symptoms include skin irritation and nausea.

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Terra firma-forme dermatosis - People with this condition appear to have patches of dirt covering their skin.

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Chronic orchialgia - Also known as chronic testicular pain, this disorder does not have any apparent cause and extreme pain must be present for more than three months to fit the diagnosis.

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Burning mouth syndrome - There is no known cause of this condition but it is characterized by chronic pain in the cheeks, gums, and lips.

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Schizophrenia - Although this mental illness continues to gain awareness, scientists are still unable to explain the cause. It is estimated that 1 in 100 people suffer from the condition, which can cause hallucinations and delusions.

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Porphyria - This disorder affects the skin, nervous system, and abdomen. It is also known as the vampire disease because it causes extreme sensitivity to the sun.

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Möbius syndrome - This neurological disorder is caused by underdeveloped cranial nerves that result in facial muscle paralysis. People with the condition cannot smile, frown, or move their eyes from side to side.

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Foreign accent syndrome - People with this condition develop speech patterns that make them appear to be talking with a foreign accent.

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Cotard delusion - A person with this mental illness believes that they are dead and do not react to external stimulation. According to The Telegraph, a British man known as Graham was diagnosed with this illness.

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Methemoglobinemia - A person suffering from this condition has elevated levels of methemoglobin (a molecule in red blood cells that delivers oxygen to the body). As a result, their skin appears blue. The most widely known case is the Fugate family in Kentucky.

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Allergy to water -

People who suffer from this allergy generally exhibit extremely irritated skin. According to the BBC, a woman named Rachel Warwick is known to have the condition.

See also: Everything you need to know about chronic fatigue syndrome

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