What happens after HIV infects cells?

Once HIV enters the body and infects cells, it replicates rapidly and spreads widely. Two to four weeks after exposure, most HIV-infected people suffer flu-like symptoms, as their immune systems fight off the initial HIV infection. This first immune response may dramatically reduce HIV levels (Pieribone, 2002/2003). As a result, the number of CD4+ T-cells in a person's body may rebound after the first, acute infection, and may even approach their original levels. The HIV+ person may then remain free of HIV-related symptoms for years.

Meanwhile, however, the virus continues replicating in the organs and tissues of the body's lymphatic system, which is the system that produces, stores, and carries white blood cells to fight infection and disease. These organs and tissues include the spleen, thymus, lymph nodes, tonsils, and adenoids. Although HIV is largely confined to a person's lymphatic system, it is detectable in his or her blood.

HIV infections usually progress to AIDS for two reasons. First, HIV evades the immune system, so the body stops fighting it. Although the body's immune system is usually strong enough to wipe out most viral infections, it cannot fight what it cannot detect. HIV is a fast-mutating virus, meaning that its genetic makeup changes often. HIV's many mutations make some of its particles invisible to the body's immune system, so that they continue to replicate and cause damage. HIV can also hide in the genetic material of an infected cell, where it is shielded from the immune system's radar.

A second reason that HIV infection generally leads to AIDS is that HIV damages the immune system, so the body cannot fight it. During HIV's life cycle, CD4+ cells change, get damaged, and die. Over time, there are not enough healthy CD4+ cells to defend the body. Without enough CD4+ cells, the body's immune system is unable to defend itself against many infections. When the immune system is so weak that it can no longer defend against opportunistic diseases, a person is said to have AIDS.