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 Now that paper has been withdrawn.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