Carriers of the AIDS virus become more infectious with time and increase their ability to pass on the infection, researchers reported yesterday at the Third International Conference on AIDS.

This may help explain why there have been fewer cases among women and suggests that the rate of heterosexual transmission might increase over the next few years, they said.

Until now, experts had thought that carriers were most likely to pass the virus on immediately after they had been infected. Researchers also believed that it was much more difficult for women to pass the virus along to men through sexual intercourse.

"There are some very unpleasant implications of these findings for heterosexual transmission," Dr. H. Hunter Handsfield of Seattle's Health Department said yesterday at a forum on heterosexual transmission of the AIDS virus. "It might just be that women {have not been infected long} enough to pass the virus on."

Although the overall number of heterosexual cases of acquired immune deficiency syndrome remains relatively small in the United States, officials from the federal Centers for Disease Control said yesterday that the rate at which cases are increasing among heterosexuals is now twice that of homosexuals.

The new data was reported by Dr. James Goedert of the National Cancer Institute. The findings are of concern to those studying the natural history of the disease, because they offer possible evidence that women can spread the virus as easily as men, as is the case in Africa, where AIDS has hit hardest.

Goedert's study examined 24 male hemophiliacs and their sex partners. All the men were carriers of the virus or had developed the disease. The study found that as the hemophiliacs showed a drop in special immune cells, called T4 cells, that ward off disease, they were much more likely to infect their partners with the AIDS virus.

Among 18 couples who did not use condoms, four of the hemophiliacs' spouses became infected. Goedert said there was a significant relationship between the number of immune cells and the point at which parters became infected. The fewer there were, the more likely the infection would be passed.

Because the AIDS virus was introduced into the American population largely through the male-homosexual community, men have been infected in greater numbers and over a longer period of time than women, researchers said.

Only 4 percent of all AIDS cases reported so far to the CDC have been classified as heterosexual contact cases, but in the last year they have shown the largest increase of any AIDS category.

Last year, 300 such cases were reported to CDC as compared to a total of 130 in 1985. That represents an increase of 131 percent. As of last week, more than 35,000 AIDS cases had been reported since the disease was first identified in 1981.

The CDC uses a ranking of risk groups. If more than one -- such as drug use and heterosexual contact -- applies, the individual is assigned to the larger risk group. In that way, many experts say, some people who contracted the disease through heterosexual contact have been assigned to other risk groups.

The Public Health Service estimates that 1.5 million to 2 million Americans have been infected with the virus, and several of this week's reports suggested that the disease will spread far beyond initial groups of homosexual men and intravenous drug users. In attempting to communicate the threat of the disease, AIDS researchers have sought to move away from placing those most at danger of infection in "risk groups" and have started to emphasize instead risk behavior.

"It does no good to talk about risk groups anymore," said Dr. Peter Piot, who reported a study of hetersexual transmission of AIDS among Belgians working in Africa. "People get on planes and travel easily. Everybody now belongs to a risk group."

Among factors that appear to increase the chances of infection, scientists said, were a history of genital lesions and past exposure to hepatitis B virus. Anal receptive sex was much more likely to result in infection than vaginal or oral sex.

Researchers said it was impossible to estimate the chances of contracting the infection after only one or a few encounters. And they hastened to remind the audience that AIDS among heterosexuals was still overwhelmingly a disease that afflicts intravenous drug users and their sexual partners.

"In New York City, which accounts for a third of the nation's AIDS cases, this is a not a casually contracted heterosexual disease," said Rand Stoneburner of the New York City Health Department. "Incidence of HIV infection is almost completely contained within known risk groups."

He and others said that those likely to be hit hardest by the epidemic are the poor blacks and Hispanics who inhabit many of the nation's inner cities.

"The typical person at risk here is not the 35-year-old suburban woman who gets her exercise at aerobics class," said CDC's Timothy Dondero. "It is the girlfriend of the black, urban drug addict."

Don DesJarlais, deputy director of the New York State Division of Substance Abuse, said that New York City officials expect to see 40,000 to 50,000 AIDS cases by 1991.

At a morning meeting DesJarlais presented the results of a study that showed that AIDS-related deaths among IV users in New York City, which has one of the best surveillance systems in the country, may be dramatically underreported. In New York City, 87 percent of heterosexual cases are sexual partners of IV drug users.

The study examined deaths among addicts from 1978, when the AIDS virus probably first appeared, to 1986. The analysis shows a dramatic increase in addict deaths which are probably related to AIDS. New York City health officials report that 1,500 addicts have died of AIDS since 1981; DesJarlais said the figure is probably closer to 3,000.

For example, in 1980 seven New York City addicts died of pneumonia other than pneumocystis, the type associated with AIDS. Last year there were 169 such deaths. In 1980 two addicts died from endocarditis, or heart infection. Last year there were 58 deaths.

"Clearly these represents epidemic increases in fatalities" that are not currently considered related to AIDS. "We must face the possibility that we are capturing only one-half of infections caught by the surveillance definition of AIDS."

Other studies showed that middle-income prostitutes were less likely to be carriers of the virus than their poorer inner-city counterparts, where high infection rates may be due to drug abuse.

One study showed that of inner-city prostitutes in Miami, 41 percent of 90 women were infected with the virus. None of the prostitutes working for an up-scale escort service were infected, according to study director Dr. Margaret Fischl.

The CDC reported that 56 percent of prostitutes in northern New Jersey, where drug abuse is common, were infected, while the rates were very low in Las Vegas, where drug use was not reported. Staff writers Sandra G. Boodman and Susan Okie contributed to this report.