Chronic fatigue syndrome has baffled scientists way before researchers defined the debilitating disorder in 1988. Long derided as the "yuppie flu" because of its high rate of incidence among young white-collar workers, the syndrome causes muscle weakness, extreme fatigue that cannot be improved by bedrest, and impaired concentration. Its cause has never been clear, and many have suggested that CFS is a mental illness.

But in recent years, the evidence has been mounting that CFS has a physical cause; to that end, it's now also known as myalgic encephalopathy. Now, a group of Norwegian scientists think they have found that cause—and it seems to be linked to the body's immune system response.

A series of small trials of an autoimmune disorder drug suggest that chronic fatigue symptoms might be caused by antibodies the body produces to fight off infections. The drug, rituximab, used to treat diseases like rheumatoid arthritis, destroys the white blood cells known as B cells. In the latest study, two-thirds of the 29 chronic fatigue patients who took the drug experienced alleviated symptoms. After three years, 11 patients were still in remission.

The researchers say the disorder could be the result of the body’s immune system getting out of whack after an infection. One hypothesis floated by Øystein Fluge, one of the study’s authors, is that antibodies produced to fight an infection might continue attacking the person’s tissues, preventing the blood from fully circulating and providing the body with oxygen.

Another recent study led by Columbia University supports the idea that CFS patients have significant differences in their immune systems from healthy subjects. This points toward a vital treatment for a little-understood disorder. A larger, better-controlled follow-up to the Norwegian study is now underway.

[h/t: New Scientist]