How does HIV cause illnesses?

HIV reproduces continuously in the body from the first day of infection. A person who is infected with HIV will typically produce about 10 billion new HIV particles each day, and about 2 billion virus–fighting immune system cells—CD4 T cells—are produced and destroyed.

A person's immune system attacks HIV soon after infection, and at first is able to clear a large amount of virus from the body every 24 hours. However, for each virus particle cleared, at least one new one is created. The body's initial, vigorous anti–HIV response creates a temporary equilibrium between immune cells and the virus that may last for months or years.

Typically, a person will show no outward signs of illness during this time, except for severe flu–like symptoms after the initial infection as a sign that the immune system is kicking–in to fight off HIV.
Over time, however, the virus gains the upper hand. The amount of HIV in the body (viral load) increases and the CD4 T cell count declines.

The immune system cannot work properly under constant attack from HIV. Eventually, the virus overwhelms the defenses of the immune system, which then can no longer ward off other illness–causing infections, some of which can be life threatening.