The search for a vaccine to prevent AIDs began in the early 1980s and proceeds at an accelerated rate today. AIDS (acquired immunodeficiency syndrome) is caused by a virus known as HIV (human immunodeficiency virus), which can exist within the body for years after exposure before causing serious symptoms of AIDS. HIV is transmitted by blood and body fluids such as semen. The disease is particularly frightening because it impairs the body's immune system, leaving it open to opportunistic infection by many other diseases. At pre- sent, because no proven cure exists, AIDS is more often than not fatal, although taking certain combinations of drugs, often called "drug cocktails," has shown some promise in reducing the symptoms and extending the lives of those infected.

Another factor that makes AIDS frightening is that it is spreading rapidly...