New evidence that a St. Louis teen-ager died of AIDS in 1969 suggests that the AIDS virus may have been introduced into the United States several times before touching off the current epidemic, according to experts in disease transmission.
Until now, many experts have assumed that the virus that causes acquired immune deficiency syndrome first appeared in the country sometime in the mid-1970's. Evidence indicates to many experts that the disease originated before then in Africa, although this has not been proved.
The patient, identified only as Robert R., died in 1969 of an illness that baffled his doctors at Washington University in St. Louis. They published a paper in 1984 suggesting that, with hindsight, his symptoms resembled those of AIDS. About two months ago, molecular biologists at Tulane University in New Orleans examined stored specimens of Robert R.'s tissues for signs of the AIDS virus and found that the 15-year-old was apparently infected with it.
''Our diagnostic tests confirmed the presence of the AIDS virus,'' said Dr. Robert Garry of Tulane. ''We're pretty confident about this case now.'' Speculation on Significance
Press accounts last weekend about the finding led AIDS experts to speculate on its implications.
The evidence that Robert R. died of AIDS in 1969, nearly a decade before what had been the country's first known AIDS cases, indicates that the virus may have been introduced and re-introduced into the American population on several occasions, but that it may have died out for lack of a large, very sexually active population to transmit it, said Dr. Richard Rothenberg, an epidemiologist at the Federal Centers for Disease Control in Atlanta.
The virus spreads through sexual intercourse or infected blood's entry into the body. Since it is not transmitted in every act of intercourse by a virus carrier, experts said, the disease might not spread widely from a small number of carriers unless they engaged in frequent intercourse with a large number of partners, a condition that was met among male homosexual populations in some cities in the 1970's.
When the virus became entrenched among addict populations - exactly when this happened is also unknown -sharing of needles was an even more efficient means of spread.
Dr. Robert May, a mathematician at Princeton University who has studied the spread of AIDS, remarked, ''It wouldn't be surprising if AIDS appeared once, twice, three times before it finally took.'' Origin of Virus
Dr. Robert C. Gallo of the National Cancer Institute, an expert on the disease, said he felt certain that ''the AIDS virus didn't come in one bout at one time.'' Cases like that of Robert R. are to be expected, said Dr. Gallo, who said he was convinced the virus originated in Africa.
Robert R.'s former doctors suspected that he had engaged in homosexual intercourse. But none of the experts had any idea where he could have become infected.
''It seems odd to me that it was in St. Louis to begin with,'' said Dr. Harold Jaffe, chief AIDS epidemiologist at the Centers for Disease Control. He noted that St. Louis was not one of the first cities to be hit by the AIDS epidemic, which was first detected in New York and California.
In 1968 Robert R. appeared at a clinic associated with Washington University suffering from an assortment of illnesses. Most striking, said Dr. William Drake, a St. Louis pathologist who is now retired, were swollen lymph nodes in Robert R.'s neck and ''swelling of the legs, lower torso and genitalia for no apparent reason.''
Dr. Drake said Robert R.'s physicians tried unsuccessfuly to treat him by surgically draining his lymph nodes.
Although the St. Louis doctors tried for 15 months to help Robert R., his disease followed an unremittingly downhill course. He was exhausted, he lost weight, and he was plagued with a severe infection with chlamydia, a bacteria that frequently infects gay men and that is sexually transmitted. His physicians treated him with a battery of antibiotics, but the youth died in 1969 after a bout with bronchial pneumonia, Dr. Drake said. AIDS-Linked Cancer Found
An autopsy showed that the Robert R. had Kaposi's sarcoma, a skin cancer that is almost a hallmark of AIDS infections in gay men. The youth had just one outward sign of the cancer, a tiny purple spot on his thigh, Dr. Drake said. But when Dr. Drake performed an autopsy, he found other Kaposi sarcoma lesions throughout the soft tissues of the youth's body.
Dr. Memory Elvin-Lewis, a chlamydia specialist at Washington University, said she was fascinated by Robert R.'s illness and wanted to study his tissues to determine the extent of his chlamydia infection. When the autopsy was done, Dr. Elvin-Lewis requested that tissues from the body be frozen so she could examine them at a later time.
Several of Robert R.'s doctors, who had since moved to the University of Arizona College of Medicine in Tucson, later suspected that the teen-ager had AIDS. Robert R. had admitted to being sexually active, although not to being gay, but his doctors said they thought he was homosexual or bisexual because he had certain rectal lesions and chronic hemorrhoids, which are frequently seen in gay men.
But Robert R. gave no hint of his sexual contacts, according to Dr. Elvin-Lewis. ''He was not communicative,'' she said. ''He barely said boo. He never told us what he was doing.'' His doctors said they doubted that he had ever left the St. Louis area.
When tests for the presence of the AIDS virus became available a few years ago, Dr. Marlys Witte of the University of Arizona, one of Robert R.'s doctors, tried to contact Dr. Elvin-Lewis to ask for the frozen tissue samples. But Dr. Elvin-Lewis said she was out of the country for much of the time that Dr. Witte was trying to reach her.
Finally, about a year ago, Dr. Witte reached Dr. Elvin-Lewis, who supplied the tissue samples. Last spring, Dr. Garry of Tulane agreed to do the tests.
Dr. Garry said he had done Western blot tests on Robert R.'s serum. The procedure is a a highly precise test for AIDS virus antibodies. Dr. Garry has also completed tests for the P24 antigen, a virus protein that gives futher evidence of infection. Both tests were positive for the HIV virus, which causes AIDS. .
While the evidence of AIDS virus infection seems virtually conclusive, Dr. Garry said, he also plans to examine Robert R.'s cells for the actual DNA, the genetic material, of the AIDS virus. But that is an uncertain and difficult procedure, he said.