rare disease
(rayr dih-ZEEZ)
A disease that affects fewer than 200,000 people in the United States. There are about 7,000 rare diseases. An estimated 25 million to 30 million Americans are living with a rare disease. The cause of many rare diseases is unknown, but they are often caused by changes in a person’s genes or chromosomes. Rare diseases are often more difficult to diagnose and treat than the more common diseases. Also called rare disorder.