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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.
Also called an orphan disease, rare diseases are those that are extremely uncommon and often have such low prevalence that a common doctor would not run into more than one case of that disease over a course of years, if ever. Here is a list of 10 diseases that rarely affect people.