According to the Atlanta-based Centers for Disease Control and Prevention, a rare disease is defined as a condition that affects fewer than 200,000 people.
» RELATED: What is rhabdomyolysis? Workout, rare condition sends teen to hospital
Globally, about 400 million people are affected by rare diseases and 25 million are impacted in the U.S. alone.
Of the approximately 7,000 rare diseases known today, about 80 percent are genetic, and about one-half of all rare diseases affect children, according to the CDC.
You may know of some rare diseases like Huntington disease or cystic fibrosis.
» RELATED: American woman diagnosed with rare disorder after waking up with British accent
But here are six you’ve probably never heard of:
Kuru
This rare neurodegenerative disease originated in the Papa New Guinea Fore tribe. Prior to the 1930s, members contracted the fatal disease by eating brains of dead family members as part of funeral rites. According to NPR, in many villages, when a person died, their remains would be cooked and consumed as an act of love and grief.
Fibrodysplasia Ossificans Progressive (FOP)
According to the National Institutes of Health (NIH), FOP causes muscle tissue and connective tissue such as tendons and ligaments to be gradually replaced by bone, ultimately forming bone outside the skeleton. At birth, those with FOP often notice malformation of the big toe.
Hutchinson-Gilford Progeria
This genetic condition, which affects about one in every 8 million children, causes children to age quickly at approximately nine to 24 months of age, according to the NIH. They also develop a disproportionately small face in comparison to the head.
» More about Hutchinson-Gilford Progeria
Von Hippel-Lindau (VHL)
According to the National Organization for Rare Disorders (NORD), VHL is the leading hereditary cause of kidney cancer. Those affected may develop tumors—mostly benign—in up to ten different areas of the body. But if tumors develop in the kidney and pancreas, the cancer can spread to the rest of the body.
Fragile X (FXS)
According to the CDC, this genetic disorder is caused by changes in the FMR1 gene, which usually makes a protein needed for normal brain development. Those with FXS, however, don't make this protein and end up with a range of developmental problems including learning disabilities.
Harlequin-type ichthyosis
Harlequin ichthyosis, a rare genetic skin disorder that affects infants before birth, causes infants born with the disorder to be covered in thick plate-like skin that can crack and split apart. These thick plates can not only distort facial features, but they can also make it harder for the infant to breathe and eat, according to the NORD.
» More about Harlequin-type ichthyosis
To learn more about rare diseases, visit rarediseases.org.
This story has been updated. A previous version incorrectly included Crohn’s disease as a rare disease.
About the Author
