expressed during this phase and that little or no viral replication was taking place—and to some degree that may still be true.

More recent data indicate a slow and gradual decline in the number of helper T-cells during the asymptomatic period. Currently, many researchers think that while most virally infected cells may not be actively producing new virus, some proportion of infected cells are. But at this stage an individual's immune system is still strong enough to eliminate a sufficient amount of the free virus and infected cells, so that the individual remains healthy.

The fact that most individuals can remain healthy for 10 years suggests that the immune system has a tremendous excess capacity, and significant numbers of T-cells can be eliminated without causing the host any severe difficulties. Eventually, however, the number of helper T-cells killed brings the HIV seropositive individual to a point where he or she can no longer handle the viral load, and symptoms of disease start to appear. Since the helper T-cells are the commander of the immune army, killing them can potentially inactivate the entire immune defense system.

T-cell Mystery

While the scenario depicted above seems logical, one great mystery remains unanswered. Scientists still cannot explain why so many helper T-cells are killed. They still do not know whether the virus kills host cells directly or whether some indirect viral activity kills the cells. Even if they did know the mechanism for viral killing, there is one nagging statistic they still must explain.

Healthy individuals have 550 to 1200 helper T-cells in each microliter (one-millionth of a liter) of blood, with the average falling around 800 cells per microliter. Individuals are defined as having AIDS if they are HIV seropositive and have fewer than 200 helper T-cells per microliter of blood. That means that a person with AIDS has lost 75 percent of his or her helper T-cells. Yet when scientists try to count the number of helper cells in which they can actually detect the virus, they find HIV in less than 1 percent of cells. How can so many cells be killed when so few seem to be infected? To solve this puzzle, scientists have proposed that HIV has some indirect means of affecting healthy immune cells in such a way that they become inactive or are targeted for immune destruction.

Shedding

So many things are going on between the immune cells and the virus that infects them that it is difficult to sort out even the most direct means



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