SUPPOSE A PERSON who knows that he has the AIDS virus persists in having sexual contact with others, thereby threatening their lives. Should that be a crime? If so, is it enough that the second person has been put at risk, or must there be actual transmission of the virus for the crime to occur? If risk is the triggering element, and to some extent lack of warning, should the law distinguish between homosexual contacts, where the risk is known to be great, and heterosexual, where it is not? And if AIDS is transmitted, what of civil liability?

These complex issues are beginning to arise in courts and legislatures, and not just here. The Soviet Union has announced that a person with the virus who continues to have sexual contact with others faces up to five years in jail. The price rises to eight years if the virus is transmitted. Your first reaction might be that this is boot-heel law that could never happen here; we care too much for civil liberties. There's the further tempting thought that some of the people who would surely favor such a law here are among the Soviet system's most vociferous critics.

But at least the first part of this is wrong. It turns out that several states, including Florida and Idaho, have already made it a crime for a person with the AIDS virus knowingly to expose another to it. Nevada has a law requiring persons arrested for prostitution -- normally a misdemeanor -- to be tested for AIDS. If those who test positive are ever arrested for prostitution again, they can be charged with a felony. The Army is court-martialing a soldier who knew he had the virus and still had sexual relations with two other persons, one a woman and one a man.

AIDS is a terrible disease. Because it invariably kills and because it is mostly spread through homosexual activity, there has been a nasty mix of hysteria and vengefulness in some of the national reaction to what must still be treated as a public health problem. But it is not hysterical to penalize as a crime the willful exposure of unknowing people to this virus. It is likely to be no more of a deterrent than any other criminal penalty, but it is basic, ordinary justice.