Sex and AIDS are on the mind of America.

One fifth of American adults say they have changed their sexual behavior in some way to avoid contracting acquired immune deficiency syndrome, a Washington Post-ABC News poll found earlier this month. Of those who are 35 or younger and never have been married, more than one third have altered their life styles to avoid AIDS.

Even those in marriages are afraid. The poll found that 14 percent say they have made changes in their sexual behavior because of AIDS.

The fears are understandable: AIDS is deadly -- it already has killed more than 19,000 of its 33,000 American victims since 1981. An estimated 1.5 million others are infected and now carry the AIDS virus without symptoms. It spreads by sexual contact, both homosexual and heterosexual, as well as through contaminated blood and shared hypodermic needles.

And, as the country well knows, there is no cure. Scientists meeting at the National Institutes of Health last week predicted that a vaccine will not be ready for general use before the mid-1990s. On Sunday, Surgeon General C. Everett Koop said a vaccine may not be ready in this century. That leaves only one way to stop AIDS: prevention.

Earlier this month, the federal Public Health Service launched its newest prevention initiative, which focuses, among other things, on making sex safe -- or at least safer.

"Now that we have taken steps to protect our blood supply, we will be concentrating on preventing the sexual transmission of the AIDS virus," Dr. Robert E. Windom, assistant secretary for health, told the House subcommittee on human resources and intergovernmental relations two weeks ago.

The federal recommendations include two options certain to prevent the spread of AIDS -- abstinence, and a faithful monogamous relationship with an uninfected partner. But recognizing that not everyone will follow that advice, the government also recommends use of condoms.

"Total abstinence from all sexual relations is one answer, but it is totally unrealistic," Koop told the National Religious Broadcasters last month.

Although much remains to be learned about the AIDS virus and the way it spreads, much already is known. The following discussion is based on interviews with medical experts. It does not address the moral aspects of sexual activity -- issues that each individual encounters in making choices about sexual behavior. Rather, it is a frank look at the evidence medical science has accumulated on ways to avoid becoming infected with AIDS. ::

Q. Is there really such a thing as safe sex?

A. Long before AIDS, there have always been risks associated with sex -- whether it was other sexually transmitted diseases or an unwanted pregnancy. AIDS, because it is lethal, has heightened fears of the potential hazards of sexual activity. "We prefer to call it 'safer' sex," said Dr. Michael Lane, director of the Centers for Disease Control's Center for Prevention Services. "There ain't no such thing as safe sex, as long as there is AIDS. We can make sex virtually safe, but not totally safe."

Q. What are the chances that a potential sex partner is carrying the AIDS virus?

A. This is difficult to estimate. One crude CDC calculation says that in a high-risk area -- such as the District -- one in 30 men, and one in 75 women, are infected with the AIDS virus. This number, however, includes people in high-risk groups -- homosexual and bisexual men and intravenous drug abusers -- skewing the estimate for people in the general population.

Therefore, in sexual encounters in low-risk groups -- such as people who are exclusively heterosexual and don't use intravenous drugs -- the chance that a partner will be a carrier are much lower.

Public health officials say that everyone -- both heterosexuals and homosexuals -- should consider themselves at risk for becoming infected with the AIDS virus through a sexual encounter.

Q. How is the virus passed from one person to the next?

A. AIDS virus can spread from one person to another only by close intimate contact that allows the exchange of bodily fluids, such as blood, semen or vaginal secretions. It cannot spread

through casual contact with a person, such as shaking hands, or by handling the objects -- a drinking glass, for example -- an infected person has touched. The virus does not travel through the air like the common cold virus.

The virus also spreads through contaminated blood, such as in transfusions, and when intravenous drug abusers share contaminated needles. The main route, however, is through sexual relations. Q. Where does the virus go when it gets in the human body, and what does it do once it gets there?

A. The AIDS virus is a retrovirus known as human T-cell lymphotrophic virus type 3 (HTLV-3), or, increasingly, as human immunodeficiency virus (HIV). It infects a variety of cells in the body, but the principal victims are T-cells, the white blood cells that marshal the body's defenses against invading microorganisms such as viruses and bacteria. HIV becomes an integral part of the T-cells' genes, causing a permanent infection. Once the retrovirus gets into the body, the individual is infected for life and is presumed to be a carrier who can pass on the infection to others.

HIV causes disease by destroying more and more of the white blood cells until the body is unable to defend itself against a wide range of strange infections and rare cancers. There also can be brain damage unrelated to the destruction of blood cells.

Q. The AIDS virus has been found in saliva. Can kissing cause AIDS?

A. The AIDS virus "is rarely found in saliva," said Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases in Bethesda, a leading center of AIDS research. One Boston study of some 50 individuals with AIDS detected the virus in only one saliva sample.

Although it would be theoretically possible to transmit the virus in saliva through casual kissing, medical experts say the possibility is remote. Erotic kissing, or "French kissing," may carry more risk.

"You can presume that it probably is difficult to transmit it by kissing," Fauci said. One problem in assessing the risk is that researchers have not been able to separate kissing from other sexual risk factors because erotic kissing seldom occurs without other forms of sex, and other sexual acts usually include passionate kissing.

In the absence of any proof one way or the other, Fauci said, "health officials have to presume that it is possible to transmit the virus by exchange of saliva in deep kissing. That presumption is made to be extra safe."

A study of 450 non-sexual family contacts -- including ordinary family kissing -- in which one member of the family is infected, failed to demonstrate the transmission of the virus, said Dr. Gary R. Noble, AIDS coordinator for the Public Health Service.

Q. The AIDS virus also has been found in tears. What is the risk that tears can transmit the disease?

A. "None. Zero. I would discount the tears thing right off," Fauci said. No study has shown that AIDS can be transmitted by contaminated tears.

Q. How does semen transmit the disease?

A. In addition to containing sperm cells, semen contains a variety of other materials, including white blood cells. Since the AIDS virus infects certain white blood cells, it can be present in the semen in significant amounts. Once infected semen enters the body of an uninfected partner, there are a number of ways it may get into the blood and cause an infection.

Q. How is the AIDS virus transmitted from male to female in vaginal intercourse?

A. It does not appear that the virus is able to infect the first layer of cells lining the vagina. But if there are sores from other sexually transmitted diseases, for example, the virus can get through the vagina's protective lining at that point and get into the blood stream. It also is possible that the virus enters the female urethra and gets across the urethra's thin mucous membrane through tiny cracks and reach capillaries -- very small blood vessels. Once the virus gets into the capillaries, it can travel throughout the blood and cause infection. Q. Are women at increased risk of becoming infected during menstruation?

A. Because blood cells could be directly infected by the virus, the risk is theoretically greater, Fauci said, but it never has been proved.

Q. Can AIDS be transmitted to men by vaginal intercourse?

A. Absolutely. The AIDS virus has been found in vaginal secretions, but the concentration is probably lower than in semen.

Heterosexual transmission from a woman to a man has been demonstrated. In one case, a 33-year-old Cleveland woman was infected with the AIDS virus by her bisexual husband. After her husband died of the disease, she became sexually involved with her 26-year-old neighbor and gave him the disease.

How the AIDS virus in the vaginal secretions infects the man is not proven, Fauci said. Researchers speculate that infected white blood cells in the vaginal secretions -- or in any other discharge caused by menstrual flow or by infections of the female's reproductive tract -- come into contact with the unprotected penis and may be absorbed through the urethra.

The AIDS virus would enter the man's body more efficiently if there are cracks or breaks in the cells lining the urethra or the skin of the penis itself.

Q. What about the risks of anal intercourse? Is it more likely to spread the virus than vaginal intercourse?

A. Unprotected anal intercourse is the main way the AIDS virus is transmitted among homosexual men. "In gay men, 95 percent or more of the infections occur from receptive anal intercourse," said Dr. B. Frank Polk, director of the Johns Hopkins University's component of the Multicenter AIDS Cohort Study.

"The data shows very clearly that for females engaging in heterosexual activity with a man, anal intercourse is much more risky than vaginal intercourse," said Polk. The risk of transmitting the AIDS virus is so great that many experts flatly advise against anal intercourse. "For women, avoid anal intercourse," said Polk. "It is the same message for gay men. Don't do it, or be sure the insertive partner wears a condom."

Polk also noted that 3 to 4 percent of AIDS virus infections have occurred in men who only act as the insertive partner during anal intercourse.

A British study, according to the Public Health Service's Noble, concluded that receptive anal intercourse was the most dangerous sexual practice followed by insertive anal intercourse and then followed by vaginal intercourse. Exactly how much the danger varies has never been measured.

Q. Why is receptive anal intercourse so dangerous?

A. Studies by Dr. Malcolm Martin of NIAID suggest that the AIDS virus can directly infect the cells of the rectum and colon and then spread throughout the body. If there is tearing of tissue during anal intercourse that causes bleeding, the virus can also spread through the blood system.

Q. Can the virus be transmitted through oral sex?

A. NIAID's Fauci said that this is a possibility, since the virus is found in semen and vaginal secretions. Such a risk would be greatest if there are cuts in the mouth and gums. But Johns Hopkins' Polk said: "We have been unable to identify a single infection that we can attribute to oral sex in gay men. Oral sex among heterosexuals has not been shown to confer risk of transmission for either partner."

But, Polk cautioned, "the fact that it has not been shown does not mean that it does not occur."

As is the case with assessing the risk of erotic kissing, it is often difficult to separate oral sex, stastically, from other forms of sex. At this time, because the risk of oral sex remains unclear, many experts advise against allowing semen to enter the mouth.

Q. Are there any sexual activities that do not transmit the AIDS virus?

A. Yes. Mutual masturbation, for example, where body fluids are not exchanged between partners, will not transmit the virus, several experts said.

Q. What is the probability that a single, unprotected sexual encounter will transmit the AIDS virus?

A. Although there are known cases in which a person became infected after a single sexual encounter with an infected individual, experts are not yet able to assign a statistical probably to any individual encounter.

One study in San Francisco concluded that there was a 9 percent risk of transmitting the AIDS virus during a single sexual encounter among homosexuals practicing unprotected anal intercourse, in which the infected individual was the insertive partner and the uninfected individual was the receptive partner. "That," Polk said, "is as close as anyone has come to predicting the transmission efficiency." The actual risk in such an encounter might be as low as 5 percent or as high as 25 percent.

Q. Are there times when people who carry the virus are not able to infect their partner?

A. Dr. Ward Cates, director of the CDC's sexually transmitted disease section, said it appears that infected individuals "shed" virus, which means they are capable of spreading it, 60 to 80 percent of the time. But for preventive purposes, "you have to assume it is 100 percent," he said, since there is no way to determine in advance whether or not an infected person will shed the virus. It's also not known whether an infected individual will stop shedding virus at some point in his or her life, or be infectious permanently. Q. What is the most effective way to block the spread of the virus during sexual activity?

A. The obvious answer is avoiding the exchange of virus-contaminated fluids. As the surgeon general has recommended, the best way to stop it is to avoid sex completely, be in a monogamous relationship with someone who is not infected, or use a condom.

Q. How do condoms stop the spread of a sexually transmitted diseases like AIDS?

A. Condoms were first used by Giacomo Casanova in the mid-18th century. His condoms were made from sheep intestines, and he advocated their use for both contraception and the prevention of sexually transmitted diseases.

When vulcanized rubber was invented in the 19th century, the natural condoms were replaced with latex ones. Properly made latex condoms have no holes, according to electron microscope studies. "Quality control procedures performed by condom manufacturers are stringent, and every condom is tested for holes," Koop said.

The condom acts as a physical barrier to the AIDS virus and virtually any other microorganism, from the spirochete that causes syphilis to the virus that causes herpes.

Q. How well do condoms block the spread of the AIDS virus?

A. Laboratory studies designed to simulate the stresses of intercourse have shown that the rubber condoms -- if properly used -- can physically contain the virus-infected semen and prevent it from reaching the man's partner, Noble said.

Studies of prostitutes in Kenya show that those who insisted that their clients use condoms had a much lower rate of AIDS virus infection than those whose partners didn't use condoms. A report last week from the Centers for Disease Control on 895 prostitutes in different cities across the country found that those who always used condoms did not show any signs of infection.

Q. Does that mean condoms are fool-proof?

A. No. "Condoms are not 100 percent," said CDC's Cates, "but they are better than nothing."

"Condoms clearly make sex much safer," added CDC's Lane.

It is not yet possible to predict with accuracy what the condom failure rate is in preventing the spread of AIDS. The only data available on condom failures come from studies of their effectiveness in preventing pregnancy. In this regard, studies show that condoms fail about a 10 percent of the time.

But this relatively high failure rate may have more to do with errors in using the condom than with the condom itself. "If you eliminate user error," Noble said, "condom failure is very low, perhaps only 2 to 3 percent."

Health officials caution against comparing condom failure rates for the prevention of pregnancy with the failure rate for the prevention of AIDS. A woman is only fertile during certain times of the month, so pregnancy is not possible during each sexual encounter. The AIDS virus, on the other hand, can cause an infection anytime it can get into the body. Studies of couples who have been using condoms because one of the partners is infected with the AIDS virus found that a small number of uninfected partners become infected anyway, CDC's Cates said.

In a Miami study of 45 AIDS-infected adults, reported in the Journal of the American Medical Association last month, only 12 couples used condoms to prevent the spread of the virus to the uninfected partner. Two of the partners already were infected at the time they entered the study, and of the remaining 10 who used condoms, one partner became infected.

Q. What is the proper way to use a condom?

A. Condoms have to be used whenever there is genital contact or there is a risk of spreading semen from the man to his partner. A small space should be left at the end of those condoms that lack a reservoir at the tip to contain all the semen after ejaculation. The condom should be manually held in place during withdrawal at the end intercourse to ensure that it stays on.

Q. Are the natural, animal condoms as effective as the latex condoms?

A. The surface of natural skin condoms tends to be less even than that of latex condoms. There is a chance that they will contain microscopic pores which might allow the virus through. "Clearly they are effective in stopping syphilis and gonorrhea," CDC's Lane said. "Studies need to be done to see how good they are at stopping the AIDS virus."

Dr. Robert Hatcher, director of family planning at Grady Memorial Hospital in Atlanta, said there is some evidence that some of the smaller viruses -- such as hepatitis B -- can get through natural condoms.

Q. What are the risks of breakage during intercourse?

A. For all types of condoms, the danger of breaking apart is greatest during anal intercourse.

Q. Can lubricants be used with a condom?

A. Yes, but the lubricant should be water-based, not a petroleum-based product such as Vasoline. The petroleum-based lubricants can act as a solvent, weakening the latex rubber, increasing the chance of breakage.

Q. Are people using condoms more frequently in light of AIDS?

A. The evidence is mixed. Last month, an NBC News-Wall Street Journal poll found that two thirds of single adults said they are more likely to use a condom or require their partner to wear a condom than they were before becoming aware of AIDS. "People are more interested in using a condom. Absolutely. No question about it," said Atlanta's Hatcher.

But Dr. Martha Gross, a clinical psychologist and sex therapist in private practice in Washington, said that a lot of people are still reluctant. Case studies performed by CDC scientists and other researchers tend to bear that out. In some cases, couples have declined to use a condom even when they knew one partner was infected with the AIDS virus -- consequently, the other partner often became infected.

Q. Why is there resistance to using them?

A. One problem is denial. Many individuals to not believe they are in any danger of contracting AIDS or any other sexually transmitted disease. "Some people will say, 'I have a special radar system that can tell whether someone is healthy,' " Gross said. "That's outrageous. You can't know."

CDC's Cates said denial also occurs because some people, often for cultural reasons, do not know, or do not believe, that the AIDS virus can be transmitted sexually.

Another problem is that people may not know they are infected and they therefore don't think they need to take precautions. A person can be infected with HIV and carry it for years -- potentially passing it on with every sexual encounter -- without knowing it. It can take four to five years or more for an infection to begin producing any symptoms.

Condoms also raise deep-seated fears about sexuality, especially for men, Gross said. "A lot of men are afraid that the condom will destroy their erection and interrupt the arousal process," she said. Many men have bad experiences with condoms -- the condom breaks, or the man has trouble putting it on or is laughed at, she said. Condom use has also been associated with "dirty" sex, prostitution and sexually transmitted diseases, such as gonorrhea and syphilis.

Q. How can this resistance to condoms be overcome?

A. Among heterosexuals, one factor is the woman's attitude. "A lot of women are terrified to ask their man to wear a condom," said Gross. They fear the man may lose his erection, become angry, accuse her of being infected, or become upset and walk away. "Women still use sex to hold onto a man. They do not want to risk losing him," Gross said. "They would rather jeopardize their personal health, and his, than lose him."

"The problem with condoms is that they have no erotic connotations," Gross added. "It will take a reformulation of our sexual behavior to help people feel more comfortable using these things. Unless we learn about them, we are not going to use them."

Experts recommend that people buy and examine condoms on their own before trying to use them with a partner.

Q. What about spermicides? Can they prevent the transmission of the disease?

A. Spermicides have been shown to kill the AIDS virus in the laboratory dish, CDC's Cates said. It may do that in the body as well, but has not yet been tested. At this point experts caution against relying on spermicides to block transmission of AIDS.

Q. In general, what other precautions can be taken to limit exposure to the virus?

A. "Know who you are sleeping with," said Hopkins' Polk. It is increasingly true that a partner's sexual history -- whom they have had sex with and the health of those other individuals -- has become increasingly important. A sexual partner who is an intravenous drug abuser or who frequently visits prostitutes has a much higher chance of carrying sexually transmittable diseases.

Another factor is the number of sexual partners. "The risk of infection increases with increased numbers of sexual partners -- male or female," said Surgeon General Koop. "Anyone who engages in freewheeling, casual sex is playing a dangerous game."

Q. Should people ask their partners about their sexual history or whether they have a disease like AIDS? How likely are they to be honest?

A. Many AIDS experts said it is rare for someone to confess to having AIDS just as they are about to have sexual relations. "It is very unlikely that someone is going to tell you on a first date that they had a homosexual experience," Gross said. "That is why it is so important to slow down sexually, because someone is more likely to be forthcoming with potentially embarrassing information over a period of time than in the first week, or the first date."

Q. Who should get an AIDS blood test to determine exposure?

A. Public Health Service's Noble put it this way: "If you live in an area where AIDS is high, you are sexually active and you can transmit it to your partner, then you should think about getting tested."

Q. Should people ask their sexual partners to be tested for exposure to the AIDS virus?

A. It's an individual decision. Testing at this point is voluntary. People who want to be tested should go to a center that has the three C's: coding {the individual test is given a code number, not the patient's name}, confidentiality and counseling." The Whitman-Walker Clinic in Washington is one AIDS testing center that follows these recommendations.

The AIDS test only detects the presence of antibodies -- proteins made as a part of the body's reaction to the AIDS virus. It does not detect the virus itself. The test indicates that the person has been exposed to the virus, but does not tell whether he or she will develop the disease.

Q. Can the AIDS blood test be wrong?

A. It takes six weeks to three months from the time of infection until the antibodies begin to appear in the blood, so if a person has a blood test too soon after exposure to the virus, the test may be negative even though the individual is indeed infected.

Most people develop antibodies within three months of exposure. A NIAID spokeswoman estimated that people can be reasonably sure that they are not infected if they do not develop antibodies to the AIDS virus 12 weeks after their last sexual encounter. It is, however, still possible that antibodies could show up later.

Dr. Ronald Valdiserri, director of the AIDS Prevention Project at the University of Pittsburgh, said that if someone has a negative test six months after the last sexual encounter, they will "have a fairly high degree of assuredness that {they} are not infected." But he pointed out, a small number of infected men never develop antibodies to the AIDS virus.

Q. What if a person is thinking about having a baby?

A. Surgeon General Koop now says that every woman thinking about having a baby should be tested for AIDS. There is a 90 percent chance that an infected woman will pass it to her baby, and, said Dr. Wade Parks of the University of Miami School of Medicine, the disease has been particularly deadly in children. Roughly half of all babies infected with AIDS die within the first year.

Q. What can a person expect if a blood test shows infection with the AIDS virus?

A. Many people first react as though it is a death sentence, but that is not yet clear. Studies show that 30 percent of people who are infected develop the disease over four to five years. Another 30 percent are likely to develop AIDS-related complex (ARC), a less severe form of the disease, over a five-year period, according to a San Francisco study. Scientists suspect that the percentage of infected people who eventually develop the disease could increase with time. Q. If an individual has tested positive on the AIDS test, how does that affect future sexual relations?

A. It's important, first, to let the partner know. And the more information the infected individual has about AIDS and how the virus gets from person to person, the easier it will be to answer the partner's questions. The infected person should also give the partner time to respond to the information.

Then, say health officials, it is essential to take precautions during all sexual activity.

Q. What if both partners test positive for the AIDS virus? Are precautions still necessary? For people already infected, does the risk of developing the disease increase with repeated exposures to the AIDS virus?

A. Health official still recommend that infected partners practice "safe sex" even though they have both been exposed to the AIDS virus. It is not yet known whether repeated exposure to the virus will hasten the onset of the disease. As Dr. Lawrence A. Kingsley of the University of Pittsburgh Graduate School of Public Health said: "We do not know whether or not repeated exposure to the virus promotes a quicker immunological decline. Is it risker? Does it convert them to AIDS? Nobody knows the answer to that."

Q. Will AIDS increase people's fears about sex to an unhealthy degree?

A. Many of the fears heightened by AIDS have always been associated with sex. Researchers note that there is a lot of ignorance and confusion when it comes to sexual topics. Many Americans are reluctant to bring up the subject. "How are you going to get Americans to change their sexual behavior when they hardly can talk about sex?" said Noble.

On the other hand, Noble said, the homosexual community in San Francisco has shown a remarkable ability to changes its sexual behaviors to reduce the risk of disease.

"Surely, over time, society has shown that where death is an outcome, people do change their behavior."Larry Thompson is science editor of the Health section. More Information

The U.S. Public Health Service's National AIDS Hotline is 800-342-AIDS. For a copy of the Surgeon General's Report on AIDS, write AIDS, P.O. Box 14252, Washington, D.C. 20044, or call (301) 443-0292. Whitman-Walker Clinic AIDS Hotline: 332-5939.