E ver since 18th-century physician Edward Jenner used a dairymaid's cowpox lesions to successfully inoculate a boy against smallpox, vaccines have been indispensable in the fight against infectious diseases. It's no wonder, then, that people might turn toward a vaccine in hopes of stemming the modern-day plague of AIDS. Indeed, as a National Academy of Sciences publication "Mobilizing Against AIDS" stated last year: " . . . the only hope for halting the spread of the disease completely is widespread immunization." With an estimated 5 million to 10 million people worldwide now thought to be infected by the AIDS virus, many research groups and companies are working at a feverish pace to develop a vaccine against the deadly disease. But these researchers have some extremely difficult, if not impossible, scientific hurdles to overcome, because the AIDS virus, also known as the human immunodeficiency virus (HIV), is like no other virus for which vaccines have been developed. "It's the first time we've considered a retrovirus for a human vaccine," says John Nutter, who heads the Prevention Branch of the AIDS Program at the National Institute of Allergy and Infectious Diseases. "It's a whole new ball game." The genetic material of a retrovirus is RNA, rather than DNA; it uses its RNA as a template to make DNA, which is then inserted into the chromosomes of a cell When the virus, which can lie dormant foi long periods of time, becomes active, it can then hijack the cell's replication machinery for the life of the cell. Moreover, unlike most other viruses HIV is extremely complex genetically and has the capacity to mutate to a worrisome multitude of variants. It attacks T4 white blood cells, which are at the heart of the body's immune response. And like a wolf in sheep's clothing, it can enter the body completely hidden inside cells an ability that may make vaccination against the cellular route of infection impossible. As a result, there is no guarantee that researchers will ever be able to make a safe and effective AIDS vaccine. And whether they succeed or not, scientists as well as the community at large will have to grapple with some thorny ethical and legal problems during vaccine testing and possible distribution. In an attempt to foresee some of these problems, to guide vaccine development