Two to four weeks after exposure to the virus, up to 70 percent of HIV-infected persons suffer flu-like symptoms related to the acute infection. The patient's immune system fights back with killer T cells (CD8+ T cells) and B-cell-produced antibodies, which dramatically reduce HIV levels. A patient's CD4+ T cell count may rebound somewhat and even approach its original level. A person may then remain free of HIV-related symptoms for years despite continuous replication of HIV in the lymphoid organs that had been seeded during the acute phase of infection.

One reason that HIV is unique is the fact that despite the body's aggressive immune responses, which are sufficient to clear most viral infections, some HIV invariably escapes. This is due in large part to the high rate of mutations that occur during the process of HIV replication. Even when the virus does not avoid the immune system by mutating, the body's best soldiers in the fight against HIV - certain subsets of killer T cells that recognize HIV may be depleted or become dysfunctional.

In addition, early in the course of HIV infection, patients may lose HIV-specific CD4+ T cell responses that normally slow the replication of viruses. Such responses include the secretion of interferons and other antiviral factors, and the orchestration of CD8+ T cells.

Finally, the virus may hide within the chromosomes of an infected cell and be shielded from surveillance by the immune system. Such cells can be considered as a latent reservoir of the virus.

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