Many people with chronic fatigue syndrome are infected with a little known virus that may cause or at least contribute to their illness, researchers are reporting.

The syndrome, which causes prolonged and severe fatigue, body aches and other symptoms, has long been a mystery ailment, and patients have sometimes been suspected of malingering or having psychiatric problems rather than genuine physical ones. Worldwide, 17 million people have the syndrome, including at least one million Americans.

An article published online Thursday in the journal Science reports that 68 of 101 patients with the syndrome, or 67 percent, were infected with an infectious virus, xenotropic murine leukemia virus-related virus, or XMRV. By contrast, only 3.7 percent of 218 healthy people were infected. Continuing work after the paper was published has found the virus in nearly 98 percent of about 300 patients with the syndrome, said Dr. Judy A. Mikovits, the lead author of the paper.

XMRV is a retrovirus, a member of the same family of viruses as the AIDS virus. These viruses carry their genetic information in RNA rather than DNA, and they insert themselves into their hosts’ genetic material and stay for life.