Vincent Racaniello is Higgins Professor of Microbiology & Immunology at Columbia University, where he oversees research on viruses that cause common colds and poliomyelitis. He teaches virology to undergraduate, graduate, medical, dental, and nursing students, and writes about viruses at virology.ws.
The detection of a new virus called XMRV in the blood of patients with chronic fatigue syndrome (CFS) in 2009 raised hope that a long-sought cause of the disease, whose central characteristic is extreme tiredness that lasts for at least six months, had been finally found. But that hypothesis has dramatically fallen apart in recent months. Its public demise brings to mind an instance when a virus *was* successfully determined to be behind a mysterious scourge: the case of HIV and AIDS. How are these two diseases different—how was it that stringent lab tests and epidemiology ruled one of these viruses out, and one of them in?