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.
Theoretically, new genetic anomalies could also arise. Perhaps the rarest disease is some manmade virus stored away in a government lab. Given what we know about diseases, how they occur and how they spread, an extremely rare disease would probably have to be one of the following: 1  A genetic disease that rarely occurs. 2  A disease mostly eradicated by humans. 3  A hereditary condition confined to a particular bloodline.