AIDS (acquired immune deficiency syndrome) is a disease of the immune system. It is caused by the human immunodeficiency virus, or HIV, which attacks a number of kinds of cells but is particularly virulent toward a class of helper T cells. The virus wages a long battle with rapidly replicating T cells, but eventually the rate of cell destruction exceeds the rate of replication. The consequence is a deterioration of the whole immune response, in particular the ability of B cells to proliferate in response to antigen stimulation. In addition, there is a general failure in T-cell activation. Most AIDS patients succumb either to diseases they could have easily resisted before contracting AIDS or to certain kinds of cancer. AIDS is so deadly because it attacks our most fundamental defenses against all disease.
Because AIDS poses such a grave threat to world health, searches for a vaccine are being intensely pursued. Such searches entail unusual problems, for the AIDS virus has an unparalleled capacity to mutate and thus develop strains resistant to any vaccine. Mutations occur in the AIDS genome at a rate many times higher than in the human genome. The genetic variation is increased by the many virus replication cycles. The magnitude of the problem can be grasped by considering our experience with the influenza virus. We have never been able to produce an effective "flu" vaccine because of the great variability of the influenza virus. HIV mutates about 60 times faster than the influenza virus.