BOSTON (AP) _ Despite an all-out scientific campaign to control the AIDS virus, it continues to spread across the world, oblivious to medicines or vaccines. The best way to avoid it is to understand it. Here are some questions and answers about the epidemic:

Q: Who is at high risk for getting AIDS?

A: So far, most of the victims have been homosexual or bisexual men or needle drug abusers, and they continue to be at highest risk. Anal sex, commonly practiced by male homosexuals, appears to be an especially efficient way to transmit the virus. Drug addicts get it by sharing needles that are contaminated with the virus.

Q: What about heterosexuals?

A: The virus can be spread through ordinary heterosexual intercourse from men to women and, apparently less frequently, from women to men. Although this has clearly happened in AIDS cases, it is not certain how often it occurs. Federal experts say there is no good evidence yet of a major spread of the virus among American heterosexuals. However, many expect that heterosexual transmission of the virus will become increasingly important over time.

Q: What happens during sex to transmit the virus?

A: The virus is carried in blood, semen and vaginal fluid. The virus can enter the body through any cut or tear that occurs during sex in the lining of the vagina or anus or on the shaft of the penis. Such tears may be too tiny to see.

Q: Is any sex truly safe?

A: Yes. Sex between two people who are uninfected with the virus is safe. The best way to guarantee this is to have sex only with one person who is absolutely faithful. If both members of a couple have been completely faithful for the past five years and don't inject drugs, there is no risk.

Q: What if you cannot be sure whether your partner is infected?

A: Condoms will cut the risk. They should be used from start to finish during sexual intercourse. However, they sometimes fail, and they are no guarantee.

Q: Can the virus be spread in just one sexual episode?

A: Yes. But the risk definitely increases with repeated exposure to the virus.

Q: Is the virus spread only by people who have AIDS?

A: Definitely not. Anybody who is infected with the virus can transmit it. Most people who are infected don't know it. They are outwardly healthy. They can carry and spread the virus for many years without becoming sick.

Q: Are some people more infectious than others?

A: Probably. There is some evidence that the longer people have been infected, the more likely they are to spread the virus to their sexual partners.

Q: Can the virus be spread by kissing, shaking hands or hugging?

A: No. It also cannot be contracted from doorknobs, toilet seats, swimming pools, eating utensils or any other non-sexual, everyday contact.

Q: What about tears and saliva?

A: Even though the AIDS virus has been found in tears and saliva, there is no evidence that anyone has ever gotten the infection from contact with these fluids.

Q: Is AIDS spread in any way besides sex and dirty needles?

A: Yes. Infected women can spread the virus to their unborn babies. Donated blood is screened for the virus, but a very slight chance remains that the infection can be spread through a transfusion. However, there is no risk of getting the virus by donating blood.

Q: How many people are infected?

A: No one knows. A U.S. government estimate issued a year ago said there may be 1.5 million carriers of the virus in the United States.

Q: Will all of them get AIDS?

A: The disease simply has not been around long enough for anyone to know. One study estimates that one-third will get AIDS within seven years of becoming infected. The official estimate is that 270,000 Americans will have gotten acquired immune deficiency syndrome by 1991.

Q: Can AIDS be treated?

A: Some drugs may slow the course of the disease, but no medicine has ever been shown to cure it.

Q: Why isn't there a vaccine to prevent it?

A: Many scientists are working to develop a vaccine, but it's a difficult task. Many different strains of the virus exist. One problem will be finding a vaccine that works against all of them. None is expected soon.

Q: Is AIDS always fatal?

A: So far as is known, no one has ever recovered from AIDS.