25 Rare Disease Facts You Should Know

By Karin Lehnardt, Senior Writer

Published May 29, 2025

Fibrodysplasia ossificans progressive (FOP), also known as Stoneman syndrome, is a disease that turns muscle and connective tissue (tendons, ligaments) into bone. Surgery to remove it just stimulates more bone growth. The disease affects one in every 2 million people._[6]
Hutchinson-Gilford progeria syndrome is a genetic condition in which a person appears to age rapidly in childhood. Symptoms of aging include development of a beaky nose, thin lips, small chin, prominent eyes, and protruding ears. HGPS occurs once in 8 million live births, and those born with the disorder only live until their mid-teens to early twenties._[7]
"Alkaptonuria" or "black urine disease" is a genetic disease that turns urine a dark color. The disorder prevents the body from breaking down certain amino acids, which, in turn, creates a homogentisic acid that discolors not only urine but also cartilage, tendons, bones, nails, ears, and heart._[1]
Auto-brewery syndrome (ABS), or "gut fermentation syndrome," is a disease in which a person's intestines produce alcohol. Symptoms include intoxication, sugar cravings, bloating, and problems with concentration._[18]
Those with Cotard's syndrome, or "walking corpse syndrome," believe that they are dead or dying or that parts of their body don't exist._[15]

The disease is named after the character Alice in the famous book "Alice's Adventures in Wonderland"

Alice in Wonderland syndrome is a disease in which a patient is confused about the size and shape of their body parts, especially the head and hands. Sufferers may also experience distorted time, touch, and sound perception._[20]
Congenital insensitivity to pain (CIP) is a genetic disease in which a person's ability to feel pain is muted. Additional symptoms include decreased sense of smell and a decreased ability to sweat._[4]
A parasitic twin is a type of conjoined twin. This happens when one twin stops developing early in pregnancy, attaches to the viable twin, and continues to grow as extra recognizable body parts._[11]
Werewolf syndrome (hypertrichosis) is a condition that causes excessive hair growth anywhere on a person's body, including the face. The cause of this condition is unknown, and there is no known cure._[16]
Trimethylaminuria, or fish odor syndrome, is a disorder in which the body is unable to break down trimethylamine, which is a chemical compound that smells like a rotten or decaying fish. The compound builds up in the body, causing a strong, fishy odor in their sweat and urine._[19]
Fatal familial insomnia (FFI) is a genetic condition that causes progressively worse insomnia. The insomnia worsens to the point of coma and ultimately death._[17]
Proteus syndrome is a rare disease that causes different types of body tissues, such as bone, skin, organs, and arteries, to grow disproportionately. The most famous case was Joseph Carey Merrick (1862–1890), who was nicknamed "the Elephant Man."_[12]
In the United States, approximately 30 million people have what is considered to be a rare disease. Of these, half are children._[14]

A rare disease is also called an "orphan disease." Many rare diseases are genetic and lifelong.

The Jerusalem syndrome is a condition in which a person who visits Jerusalem becomes excessively religious. Symptoms include compulsive grooming, wearing white robes, and sermonizing. Most people without a preexisting psychiatric condition return to normal within 5–7 days._[9]
Alien hand syndrome (Dr. Strangelove syndrome) is a neurological condition that causes uncontrollable hand or limb movement._[2]
Geographic tongue is a disease in which harmless but painful lesions cover the tongue, creating irregular, map-like shapes._[8]
Porphyria, or "vampire disease," is a group of disorders that cause skin blisters and other skin issues when exposed to sunlight._[1]
Genetic mutations cause about 80% of what are considered "rare" diseases, which can then be inherited. Approximately 10% of rare diseases arise spontaneously._[21]
Rasmussen's Encephalitis, or Chronic Focal Encephalitis, occurs in about 2 per 10 million people. It is marked by severe seizures, loss of speech and motor skills, encephalitis, and mental deterioration. There are treatments to decrease brain inflammation but none to ultimately prevent disability._[3]

The zebra has become a powerful symbol for those suffering from a rare disease

The ribbon color for rare diseases is the zebra print. Medical students are usually taught the phrase, "If you hear hoofbeats, think horses, not zebras," which means they should consider common conditions first. So, patients with rare diseases are the zebras rather than the horses._[13]
One of the rarest diseases in the world is Fields' disease, a severe and progressive disease that causes muscular degeneration. There are only 2 confirmed cases, twins Kristie and Catherine Fields. Scientists are still studying the disease._[5]
Rare Disease Day takes place on the last day of February, either the 28th or the 29th—the rarest calendar day—to highlight the nature of the diseases._[13]
A disease is considered rare if it affects fewer than 1 in 2,000 people._[13]
The leading causes of infant mortality in the United States are genetic disorders and congenital anomalies._[22]
Drugs that are used to treat rare diseases are called "orphan drugs." These drugs are typically very costly to produce._[10]