There is strong new evidence that the AIDS virus may directly affect the brain and spinal cord, leading to nervous system damage in some patients with acquired immune deficiency syndrome and perhaps posing a lifetime risk for others infected with the virus but not initially ill.

Researchers say that the findings may present new difficulties in seeking a treatment for AIDS and expand the range of illnesses for which the virus may be responsible.

They also raise the possibility that people who carry the virus but do not show the pattern of life-threatening infections now associated with AIDS could end up with a debilitating or fatal brain disorder years from now. While more than 15,000 Americans have been reported to have AIDS, experts suggest that more than 1 million people in the U.S. may have been infected but show no symptoms.

Two new studies directed by researchers at the Harvard Medical School and National Cancer Institute detected the virus in central nervous system fluids and tissues from a high percentage of AIDS patients with neurological problems affecting them both mentally and physically.

Commenting on the new research in this week's issue of The New England Journal of Medicine, Boston University's Dr. Paul H. Black warns that the new findings complicate the already difficult fight against AIDS.

He notes that the brain may provide a special "sanctuary" for the AIDS virus, known as HTLV-3, on a persistent basis. It may be hard to get rid of it there because many drugs are prevented from reaching the brain by a natural barrier separating the body's blood supply from the central nervous system.

"The special feature of persistent infection in the brain, coupled with the difficulty of bringing adequate concentrations of drugs . . . across the blood-brain barrier, will make eradication of HTLV-3 infection of the central nervous system very difficult, if not impossible," Black wrote in an editorial.

Particularly troubling, but as yet unanswered, is the degree to which initial infection with the HTLV-3 virus may "seed the brains" of individuals who otherwise escape immune system damage, only to trigger chronic central nervous system disease many years later. "That's a big question," Black said.

He added that while preliminary research suggested that the AIDS virus may grow in brain tissue and cause disease, the new studies indicate convincingly that this occurs much more frequently than previously suspected.

Dr. David D. Ho, who headed the Harvard study at Boston's Massachusetts General Hospital, said his team had recovered HTLV-3 virus from 24 of 33 patients with AIDS-related nervous system disorders. Doctors had previously found such disorders in approximately one-third of patients with AIDS, but it had not been known whether the AIDS virus or other infections that often accompany the disease produced the disorders.

Ho also found the virus in four patients with nervous system disorders, but they did not show the classic immune system problems associated with AIDS.

"What this says is that the virus is capable of causing more than immunodeficiency. In the future we ought to keep our eyes open for neurologic problems" in individuals at high-risk for AIDS, said Ho.

Research by Dr. Lionel Resnick, under the auspices of the National Cancer Institute laboratory of Dr. Robert Gallo where the HTLV-3 virus was discovered, also found the virus may be actively growing in infected patients without immediately showing outward nervous system symptoms. Gallo's group had earlier found the genetic footprints of the virus in brain autopsy tissue from AIDS patients with neurological problems.

As of Dec. 2, there were 15,172 Americans reported to have AIDS, and half of them had already died. Three-fourths of the cases occurred in homosexual or bisexual men, 17 percent in intravenous drug abusers. Their sexual partners, children of infected women and those who have come into contact with infected blood in the past are also at higher risk of getting the disease.