Tennis star Venus Williams withdrew from the U.S. Open on Wednesday, citing a diagnosis of Sjögren's syndrome, a little-known autoimmune disorder that causes fatigue and joint pain… Estimates vary, and there may be a fair number of people who have Sjögren's and don't know it. So the estimates vary widely. A European study found that about 4% of the population may be affected, while a study by the Mayo Clinic found a much lower prevalence of around 1%. Additional coverage: Huffington Post, Yahoo News Canada
TIME, by Alice Park, 9/2/11
Tags: Rheumatology