Specifically, crucial immune cells, called CD4+T cells, are disabled and killed during the typical course of HIV infection. These cells, also known as T helper cells, play a central role in the body’s immune response; they signal other cells in the immune system to perform special functions.

A healthy, uninfected individual has approximately 1000 CD4+T cells per cubic millimetre (µL) of blood.

When HIV enters a person’s CD4+T cells, it uses the cells to replicate or make copies of itself. This process destroys the CD4+T cells and the CD4+T "count" goes down. As CD4+T cells are destroyed, the person’s immune system becomes weaker.

When the CD4+ T cell count falls below 200/µL, the person becomes vulnerable to infections known as "opportunistic infections" and cancers that typify AIDS.

• An opportunistic infection is an infection that is highly unusual in a person who has a healthy immune system and is not HIV positive. People with AIDS are frequently affected by opportunistic infections that attack the respiratory tract, gastrointestinal tract, neurological system, and other organs, and cancers such as Kaposi’s sarcoma and some types of lymphomas.