destroy the membrane's structural integrity. HIV's fragility in air and in the environment in general means that it cannot be passed between individuals by touching them or by touching surfaces they have touched.

The virus that leaves the cell looks pretty much like the one that entered it. The mature virus has an outer protein shell and an inner one that carries viral RNA, integrase, and reverse transcriptase. On its exterior the 20-sided viral particle is wrapped in a cellular membrane. The viral surface is studded with the lollipop-shaped glycoproteins that stand poised and ready to gain entry to another cell via the CD4 receptor. For a time the newly manufactured viral particle circulates freely in the blood, or in whatever body fluid its host cell is floating, until it encounters another CD4-bearing cell, infects it, and the whole cycle starts over until more and more cells become infected.

HOW HIV CAUSES AIDS

Since the mid-1980s when HIV was identified as the cause of AIDS, scientists have learned a tremendous amount about the natural history of the virus and the symptomatology of the disease. Yet putting these things together has proved more difficult than anyone would have imagined. As the virus's life style is understood in greater detail, it becomes less clear exactly how it decimates the immune system and causes AIDS.

The problem is not in identifying ways in which the virus's behavior is destructive to the immune system. On the contrary. The problem is that the virus is destructive to the immune system in so many ways that it is difficult to identify the one activity, or constellation of activities, that constitutes the primary cause of the destruction. The hope in identifying the primary cause is to develop therapeutics that will block that avenue of destruction and reduce the toll the virus takes on a person with AIDS.

Infection

Technically, it would be wrong to say that the disease starts at the moment of infection. There is a sharp distinction between infection and disease. Yet it is clear that infection with HIV is a necessary precondition for later developing AIDS. It is at the moment of infection that the virus has the potential of embarking on its life cycle, multiplying, and causing disease. It is also currently believed that the destruction of the immune system that ultimately leads to AIDS begins soon after infection.

But infection requires a very particular kind of contact between



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