The AIDS virus is known to be diabolical, preferentially destroying the one type of cell that is indispensable to the immune system. But it is being found to be selfish, too. Evidence shows that once a given strain of the virus takes up residence in the body, it prevents all other strains of acquired immune deficiency syndrome virus from gaining entrance.

The scientists who reported this finding in this week's issue of Science say it may hint at a new way of preventing the disease from spreading. If one strain of the AIDS virus can make the body immune to other strains, researchers may determine how this happens and evoke those barriers to infection before the first AIDS virus gets in.

So far, however, how the virus exercises its selfishness is unknown, and it will probably take considerable research to find out. Once inside, the researchers said, the virus evolves extremely rapidly, producing several new strains in each victim in the few years preceding death.

After analyzing the genes of each viral strain within a number of infected persons, they found that all the strains from any given individual were quite closely related, having few genetic differences, and were descended from a common ancestor. The differences among the strains inhabiting one body were tiny compared with the differences between the strains of one individual and those of another.

Then it dawned on the scientists. Since some victims of AIDS have had large numbers of sexual partners, they have probably been exposed to the virus many times and in many widely differing strains. Why, then, didn't any victims have AIDS viruses from more than one close-knit family?

The absence of multiple virus families in any one patient, they concluded, suggests that the first virus to invade establishes barriers against all others and then rapidly evolves its own family of variants within a body.

If such a phenomenon exists, it may explain the observation, reported last March, that a form of the AIDS virus in West Africa infects people but does not cause disease. It leaves the immune system intact. It appears that this virus also protects people against infection by the disease-causing form of the AIDS virus. If the benign virus gets in first, it blocks access by the pathogenic version.

The new research was reported by a 10-member team from four institutions: the University of Alabama, Walter Reed Army Medical Center, the National Cancer Institute and the University of Miami. The first author of the Science paper is Beatrice H. Hahn of Alabama.