A recent study released in the journal Science showed that a virus, known as XMRV, showed up in the blood of 68 out of 101 patients with Chronic  Fatigue Syndrome compared to the virus being present in only 8 out of 218 control subjects.

What is not known is whether XMRV, which is also linked to prostate cancer, actually causes Chronic Fatigue Syndrome or if it is an opportunistic virus which appears as a result of Chronic Fatigue Syndrome.

There has been a great deal of debate surrounding the definition of Chronic Fatigue Syndrome. In 1994, a group of international medical experts drafted a definition of CFS. Essentially, CFS is characterized by a profound fatigue, unexplained by other medical causes, that lasts 6 months or longer and involves swollen lymph glands, painful joints and/or muscle pain, sore throat, sleep that is not restful, and several other symptoms. The Centers for Disease Control (CDC) has useful information on Chronic Fatigue Syndrome.

The XMRV virus, known as xenotropic murine leukemia virus-related virus, has been linked to prostate cancer and is known to cause leukemia and tumors in animals. It is not unusual for viruses to be linked to specific diseases (think of HIV, hepatitis B & C, Epstein-Barr virus etc).

For more information about the link between viruses and cancer, the Proceedings for the National Academy of Sciences has a web site that has published an in-depth article about the link between XMRV and prostate cancer.