A rare disease is defined in a few ways. In Europe, a disease is rare if it affects less than 1 in 2, 000 people. In the United States, a disease is rare when it affects less than 200,000 people. There are around 6,800 rare diseases known to the National Institutes of Health. 

Eighty percent of rare diseases have genetic origins, and fifty percent affect children.

A rare disease may also be referred to as an orphan disease.