EARLIER this month, a study published in the journal Science answered a question that medical scientists had been asking since 2006, when they learned of a novel virus found in prostate tumors called xenotropic murine leukemia virus-related virus, or XMRV: Was it a human infection?

XMRV is a gammaretrovirus, one of a family of viruses long-studied in animals but not known to infect people. In animals, these retroviruses can cause horrendous neurological problems, immune deficiency, lymphoma and leukemia. The new study provided overwhelming evidence that XMRV is a human gammaretrovirus  the third human retrovirus (after H.I.V. and human lymphotropic viruses, which cause leukemia and lymphoma). Infection is permanent and, yes, it can spread from person to person (though it is not yet known how the virus is transmitted).

That would have been news enough, but there was more. XMRV had been discovered in people suffering from chronic fatigue syndrome, a malady whose very existence has been a subject of debate for 25 years. For sufferers of this disease, the news has offered enormous hope. Being seriously ill for years, even decades, is nightmarish enough, but patients are also the targets of ridicule and hostility that stem from the perception that it is all in their heads. In the study, 67 percent of the 101 patients with the disease were found to have XMRV in their cells. If further study finds that XMRV actually causes their condition, it may open the door to useful treatments. At least, it will be time to jettison the stigmatizing name chronic fatigue syndrome.

The illness became famous after an outbreak in 1984 around Lake Tahoe, in Nevada. Several hundred patients developed flu-like symptoms like fever, sore throat and headaches that led to neurological problems, including severe memory loss and inability to understand conversation. Most of them were infected with several viruses at once, including cytomegalovirus, Epstein-Barr and human herpesvirus 6. Their doctors were stumped. The Centers for Disease Control and Prevention, the nation’s presumed bulwark against emerging infectious diseases, dismissed the epidemic and said the Tahoe doctors “had worked themselves into a frenzy.” The sufferers, a C.D.C. investigator told me at the time, were “not normal Americans.”