present. Therefore, this phase of HIV disease is called the clinical latency period. Although the majority of peripheral blood T cells do not harbor the virus, destruction of CD4+ T cells within lymphoid tissues steadily progresses during the latent period, and the number of circulating blood CD4+ T cells steadily declines (Fig. 20-8). More than 90% of the body's approximately 1012 T cells are normally found in peripheral and mucosal lymphoid tissues, and it is estimated that HIV destroys up to 1 to 2 x 109 CD4+ T cells every day. Early in the course of the disease, the body may continue to make new CD4+ T cells, and therefore these cells can be replaced almost as quickly as they are destroyed. At this stage, up to 10% of CD4+ T cells in lymphoid organs may be infected, but the number of circulating CD4+ T cells that are infected at any one time may be less than 0.1% of the total CD4+ T cells in an individual. Eventually, over a period of years, the continuous cycle of virus infection, T cell death, and new infection leads to a steady decline in the number of CD4+ T cells in the lym-phoid tissues and the circulation.
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