Narod SA. Tamoxifen ChemopreventionâEnd of the Road?. JAMA Oncol. 2015;1(8):1033â1034. doi:10.1001/jamaoncol.2015.2247. Download citation file:
About Rare Diseases. A rare disease is defined by the European Union as one that affects less than 5 in 10,000 of the general population. There are between 6,000 and 8,000 known rare diseases. Around five new rare diseases are described in medical literature each week.
A rare disease is generally considered to be a disease that affects fewer than 200,000 people in the United States at any given time. Rare diseases are also sometimes called orphan diseases. How many rare diseases are there? There are more than 6,800 rare diseases. Altogether, rare diseases affect an estimated 25 million to 30 million Americans. Top of page. What causes rare diseases? The exact cause for many rare diseases remains unknown. Still, for a significant portion, the problem can be traced to mutations (changes) in a single gene. Such diseases are referred to as rare, genetic diseases. Many of these genetic mutations can be passed on from one generation to the next, explaining why certain rare diseases run in families. It is important to keep in mind that genetics are just one piece of the puzzle.