An increasing number of states now have laws that make it a crime to knowingly expose another person to the AIDS virus.

Since 1986, 22 states have passed laws making it illegal to engage in conduct that could transmit the human immunodeficiency virus HIV, believed to cause acquired immune deficiency syndrome, according to the AIDS Policy Center at George Washington University."The idea of trying to prosecute somebody for attempted transmission of HIV is increasingly, almost alarmingly, common," said Lawrence O. Gostin, director of the AIDS Litigation Project of the U.S. Public Health Service and a professor at the Harvard University School of Public Health.

But, he said, "when somebody is actually having sex with somebody, I think the risk is significant enough that prosecutors are well within their rights to prosecute."

While the number of AIDS-related prosecutions nationwide is not known, the military seems to be having the best success with such cases, the experts say.

Last week the Supreme Court rejected without comment an appeal by Nathaniel Johnson Jr., an Air Force sergeant who was convicted in a military court of aggravated assault because he had homosexual relations at McChord Air Force Base, Wash., while knowingly infected with the virus.

The Air Force Court of Military Review said at least six previous courts-martial had been convened based upon AIDS-related assaults. Such conduct, it said, "can be analogized to attempting to put poison in the drink of a victim."

The outcomes in civilian courts so far have tended to be different, Gostin said.

"It is enormously problematical to try to reach into the bedroom and create a criminal prosecution around it, and the only ones who have been successful in doing that are the military," Gostin said.

Part of the reason is because most civilian cases involve biting, spitting or splashing blood rather than sexual activity.

"It is virtually impossible - if not impossible - to transmit the virus that way," Gostin said. "It appears to me to be much more punitive than effectuating any clear public policy."

Gostin said most cases in the state courts are brought under general criminal laws rather than specific AIDS-related legislation. He said he had not found one such case in which HIV was transmitted.

James R. Klimaski, a Washington lawyer who specializes in military law, said the military has an easier time with sexual cases because it is a much more controlled environment.

According to the AIDS Policy Center, intentionally exposing someone to the virus is a felony in Arkansas, California, Colorado, Delaware, Florida, Georgia, Idaho, Illinois, Indiana, Kentucky, Michigan, Missouri, Ohio, Oklahoma, South Carolina, Texas and Virginia.

It is a misdemeanor in Alabama and Maryland. The center said Nevada, Mississippi and Louisiana passed similar laws without specifying whether the offense was a felony or misdemeanor.

The laws vary as to the circumstances in which they apply.

AIDS attacks the body's immune system, leaving victims susceptible to a wide variety of infections and cancers. It is most often transmitted through sexual contact, mostly between homosexual males.