AIDS (acquired immune deficiency syndrome) is a disease of the immune system caused by the human immunodeficiency virus, or HIV (Figure 7.37b), which attacks a number of kinds of cells but is particularly virulent toward a class of helper T cells.
T-cell Infection - HIV 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.
High Mutation Rate - Mutations occur in the AIDS genome at a rate many times higher than in the human genome, thwarting efforts to develop a cure. To better understand the magnitude of the problem, consider that a life-long "flu" vaccine has never been developed because of the variability of the influenza virus. HIV mutates about 60 times faster than the influenza virus.
Because of the difficulty in producing safe and effective vaccines, and because HIV infection has already spread so widely, huge efforts have been made to produce a therapy. The efforts that have been most successful so far have used a combination of approaches. On the one hand, the replication of the virus can be slowed by specific inhibitors of reverse transcriptase (see here and here). A quite different approach uses a protease inhibitor to block a proteolytic step essential in the maturation of new viruses within the infected cells.