of the world. Today, more than 600,000 people worldwide have been diagnosed with AIDS (see Figure 3.1), and in the United States alone at least 1 million people are infected with HIV (see Figure 3.2). Throughout the world, still more are infected and have yet to know or have yet to develop the symptoms of AIDS.

But numbers alone do not reflect the dimensions of the "world AIDS crisis," as President Clinton called it. Certainly, more people have had flu from influenza virus than have suffered from AIDS. What distinguishes AIDS from every infectious disease to come before it is the scientific challenge it represents to a research community that has successfully controlled all of the plagues of the past. Hardly anyone living in the United States today can remember a time when they were concerned about typhoid or tetanus. Few children today will grow up with the threat of polio or smallpox. Improved public health measures and potent vaccines have made these diseases the exception rather than the rule in the United States and in most of the industrialized world. In more recent memory, both toxic shock syndrome and Legionnaire's disease emerged and were vanquished by science within a few short years.

Against this backdrop of success fighting infectious diseases, comes HIV, a virus whose natural history is so unusual that no prior experience with any infectious agent has appropriately prepared modern science to tackle it. And yet no previous era has been better prepared to take on the challenge. Only now does science have the experimental tools to unravel the details of the peculiar life style of this virus. It is one of the greatest ironies of this epidemic that stopping it will owe as much to the technology of the era as its spread.

The hallmark of AIDS is the slow but complete erosion of the immune system. Years, sometimes many years, after a person becomes infected with the virus, his or her immune system is so weakened that it is unable to fight off the bacterial and viral assaults that a strong immune system could easily defeat. For that reason people with AIDS are vulnerable to a host of diseases rarely found in the general population. These are called opportunistic infections because they take the opportunity to strike someone with depleted immune defenses. Ultimately, the patient dies from the effects of one of these diseases.

The list of opportunistic infections that most frequently affect AIDS patients itself reads like a catalog of plagues. Tuberculosis, recurrent

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