When basketball hero Magic Johnson recently announced that he had tested positive for HIV, the public naturally assumed it was only a matter of time before he developed full-blown AIDS. But is the link between HIV and AIDS really established? Most physicians now believe that HIV is tantamount to a death sentence. They also believe that AIDS is a fundamentally new disease whose cause remained unknown until the discovery of HIV. That discovery was hailed as a great advance in the fight against this devastating plague, and it has been cited to justify the continuing huge expenditure of billions of dollars a year in public funds on AIDS research. But do we know that AIDS is new? Do we really know its cause? Robert Root-Bernstein, a researcher in biochemistry and autoimmune diseases, argues that AIDS is not new, and strongly criticizes the AIDS research extablishment for ignoring historical data to the contrary in their haste to declare the AIDS puzzle solved. In fact, he argues, AIDS has been around a lot longer than anyone realizes; its fundamental cause is depression of the immune system; that this can occur for many different reasons; and that the relation between HIV and AIDS may be more correlational than causal. In short, we still don't know what causes AIDS. Lifestyle theories of causation are just as plausible, given the current state of knowledge, as the HIV hypothesis. Root-Bernstein provides a thorough and authoritative, yet accessible view of the existing AIDS research, drawing on medical records to show that hundreds of cases of AIDS may have occurred in the course of the past hundred years, and presenting several plausible alternatives to the HIV hypothesis.