The virus known as XMRV does not cause chronic fatigue syndrome.
Achieving this particular bit of knowledge has taken a pretty spectacular couple of years.
In October 2009, Judy Mikovits, a scientist then at the Whittemore Peterson Institute in Reno, Nevada, and her colleagues published a startling paper. They found that 68 out of 101 people suffering from chronic fatigue syndrome (also known as myalgic encephalomyelitis) carried a virus called XMRV. Only 8 out of 218 healthy people had it. That’s 67% versus 3.7%. Mikovits and her colleagues raised the possibility that the virus played a part in the disorder, which affects an estimated 60 million people. If that were true, then there might be a straightforward way to treat people: wipe out the offending virus.