SUPPOSE THAT YOU are an employer, and you learn that one of your employees is carrying the AIDS virus. Should you fire him to protect other employees' health? The answer to that one is no. No one has ever caught the disease through normal contact in the office, shop or school. But suppose that his presence bothers you and, medical risk or not, you want to get rid of him. There's a federal law prohibiting discrimination against handicapped people. Does it prevent you from firing him? In a memorandum last month the Justice Department argued that it does not.

That memorandum has been met with a great deal of sharp rebuttal, including a column by Charles Krauthammer in this newspaper. On the op-ed page today we publish the department's response to Mr. Krauthammer. Earlier, when the memorandum appeared, we commented on the law and the balance it needs to strike. But the Justice Department is asserting a policy that has deeply troubling implications, especially the status that it would provide for irrational fears of the disease and of the people who are infected. That is the point made by Mr. Krauthammer and, on Friday, by the American Medical Association in a suit now before the Supreme Court.

The Justice Department holds that, when Congress enacted protection for handicapped people, it did not intend to include those who carry communicable diseases. The law forbids discrimination on a long list of grounds -- race, gender, age, handicap and so forth -- but, Justice says, infection with AIDS or any other disease is not on the list. Since carriers aren't protected, Justice argues, it doesn't make any difference whether people's fears of contracting the disease by casual contact are rational or irrational. And if it's not illegal to discriminate against genuine carriers, then it's not illegal to discriminate against people only suspected of being carriers.

The courts will be asked sooner or later whether the department's reading of the law is correct. Sooner would be better.

In effect, the memorandum provides a rather explicit set of directions for discriminating against not only AIDS victims but anyone who might be suspected of carrying the disease -- homosexuals in general. And perhaps, in addition, people afflicted by diseases other than AIDS. The case now before the Supreme Court involves tuberculosis. How about cancer? Until now, the discrimination statute has protected people suffering from it. The Justice Department thinks that it can distinguish between AIDS and cancer, but its logic is not compelling. An employer could claim that he feared catching an employee's cancer. Remember, Justice says that fear of the disease need not be rational.

The memorandum makes the further mistake of suggesting heavily that the danger of infection through casual contact is an open question. The same administration's Department of Health and Human Services quickly pointed out, uneasily, that Justice was not making a medical judgment and was offering no new medical findings. There are only three known ways to get the disease: sexual intercourse, direct introduction of infected blood into a person's bloodstream, and birth from an infected mother. More than 22,000 cases have been diagnosed so far, and not one of them has been shown to have been contracted any other way.