BAKERSFIELD, Calif. -- Chronic fatigue syndrome, a condition characterized by widespread muscle and joint pain and chronic, often severe, mental and physical exhaustion, is thought to affect 1 in 250 adults in the U.S.

It is estimated that only 5 percent of patients ever fully recover. While researchers still do not know what causes chronic fatigue syndrome, a new study conducted at the National Cancer Institute has found a strong link between this condition and the human virus, called XMRV.

After testing blood samples from 100 patients with chronic fatigue syndrome and more than 200 healthy patients, researchers found that two-thirds of patients with chronic fatigue were a carrier for this virus while only 4 percent of those they tested without the condition had the virus.

Researchers were able to show that this virus provoked an immune response in patients, but more research is needed to determine whether the virus is a possible cause for the syndrome.