The Institute of Medicine is reviewing how chronic fatigue syndrome is diagnosed and whether that label puts too much emphasis on fatigue over other significant symptoms. Daniel Horowitz for NPR hide caption

itoggle caption Daniel Horowitz for NPR

A virus called XMRV was linked to chronic fatigue syndrome in a study published in Science in 2009. Now that paper has been withdrawn. David Schumick and Joseph Pangrace/Cleveland Clinic Center for Medical Art & Photography hide caption

itoggle caption David Schumick and Joseph Pangrace/Cleveland Clinic Center for Medical Art & Photography

Some patients describe chronic fatigue syndrome as feeling like an "unrelenting flu." iStockphoto.com hide caption

itoggle caption iStockphoto.com

XMRV, a mouse virus, may be an artifact of laboratory experiments rather than the cause of chronic fatigue syndrome. Whittemore Peterson Institute hide caption

itoggle caption Whittemore Peterson Institute