The risk of becoming infected with the AIDS virus does not appear to correlate simply with the number of times a person has sexual intercourse with an infected partner, according to a study of infected individuals and their spouses.

In a study of 80 couples in which one spouse had acquired the virus from a blood transfusion, scientists found that one woman contracted the AIDS virus from her husband after having sexual intercourse once, while 11 other women remained uninfected although each had intercourse with her infected husband more than 200 times. The report appears in today's issue of the Journal of the American Medical Association.

"I think there's good news and bad news," said Dr. Harold Jaffe, chief of AIDS epidemiology at the federal Centers for Disease Control and one of the study's authors. "The good news is, most partners were not infected. . . . The bad news is, it wasn't predictable when transmission {of the virus} would occur."

There were 49,743 AIDS cases reported to the CDC as of Monday. Only 4 percent of adult cases occurred through heterosexual transmission of the virus, but 29 percent of the AIDS cases in women occurred through heterosexual transmission, according to CDC figures.

Among the 80 exposed spouses in the study, only 12 -- or 15 percent -- became infected with the virus, even though most of the couples were initially unaware of the transfusion-related infection and had intercourse many times without taking precautions.

Transmission occurred more frequently from husbands to wives than vice versa. Eighteen percent of women with an infected husband contracted the virus, but only 8 percent of men with an infected wife did.

The study casts doubt on previous estimates that the risk of acquiring the AIDS virus from a single sexual contact with an infected person was one percent or less. Jaffe said the risk seems instead to vary unpredictably, depending on unknown biological factors. Such factors could include the number of viral particles present in an infected individual's blood, semen or vaginal secretions, or an exposed sexual partner's susceptibility to infection.

Jaffe said that the low rate of transmission of the virus among couples in the study should not lead heterosexual adults to stop worrying about acquiring the AIDS virus from a sexual partner. "In fact, it may reinforce the need to take precautions," he said. "We've got examples of people who were infected with only a few instances of sexual contact."

Studying couples in which one member had become infected via a transfusion allowed the scientists to examine heterosexual transmission of the AIDS virus in a group that did not have other risk factors for the disease, Jaffe said. In each case, the date of the transfusion was known. Because most couples were past reproductive age and all were initially unaware of the infection, few used condoms or other contraceptives, such as diaphragms or spermicides, that might have prevented transmission of the virus.

Couples were asked to estimate how often they had intercourse and to describe other sexual habits. The number of sexual contacts per couple ranged from one to more than 1,000. Only two couples reported occasional anal intercourse, and the study showed no correlation of viral transmission with sexual behavior other than vaginal intercourse.

Of the 12 spouses who became infected, two were men and 10 were women. One of the women died of AIDS, and three developed enlarged lymph nodes. The other eight spouses had no symptoms, the study said.

Jaffe said studies of heterosexual transmission of the AIDS virus to sexual partners of hemophiliacs and of bisexual men had shown a similarly low rate of infection, in the range of 10 percent to 20 percent. But studies of heterosexual transmission to sexual partners of infected drug addicts and infected servicemen, as well as AIDS patients in Miami and Zaire, have shown much higher rates -- ranging from 42 percent to 71 percent.

He said the higher rates found in some studies might be due to other risk factors for AIDS or to exposure to more than one infected sexual partner. In addition, some strains of the virus might be more infectious than others or some individuals might "shed" more viral particles in their blood or secretions. He said some research suggests that, in an infected individual, the greatest shedding of virus may occur soon after infection and, again, late in the course of the disease.

The new study found no cases of infection in 63 other family members of infected individuals, confirming reports that failed to find any evidence for casual transmission of the AIDS virus.