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.

Dr. Mikovits and other scientists cautioned that they had not yet proved that the virus causes the syndrome. In theory, people with the syndrome may have some other, underlying health problem that makes them prone to being infected by the virus, which could be just a bystander. More studies are needed to explain the connection.

But Dr. Mikovits said she thought the virus would turn out to be the cause, not just of chronic fatigue, but of other illnesses as well. Previous studies have found it in cells taken from prostate cancers.

“I think this establishes what had always been considered a psychiatric disease as an infectious disease,” said Dr. Mikovits, who is research director at the Whittemore Peterson Institute in Reno, a nonprofit center created by the parents of a woman who has a severe case of the syndrome. Her co-authors include scientists from the National Cancer Institute and the Cleveland Clinic.

Dr. Mikovits said she and her colleagues were drawing up plans to test antiretroviral drugs — some of the same ones used to treat HIV infection — to see whether they could help patients with chronic fatigue. If the drugs work, that will help prove that the virus is causing the illness. She said patients and doctors should wait for the studies to be finished before trying the drugs.

Dr. William Schaffner, an infectious disease expert at Vanderbilt University, said the discovery was exciting and made sense.

“My first reaction is, ‘At last,’ ” Dr. Schaffner said. “In interacting with patients with chronic fatigue syndrome, you get the distinct impression that there’s got to be something there.”

He said the illness is intensely frustrating to doctors because it is not understood, there is no effective treatment and many patients are sick for a long time.

He added, “This is going to create an avalanche of subsequent studies.”