The immune system is a network of cells, organs and proteins that work together to defend and protect the body from potentially harmful, infectious microorganisms (microscopic life-forms), such as bacteria, viruses, parasites and fungi.

The immune system also plays a critical role in preventing the development and spread of many types of cancer.

When the immune system is missing one or more of its components, the result is an immunodeficiency disorder. AIDS is an immunodeficiency disorder.

Lymphocytes (white blood cells) are one of the main types of immune cells that make up the immune system. There are two types of lymphocytes: B cells and T cells. (T cells are also called CD4 cells, CD4 T cells, or CD4 cell lymphocytes).

B cells secrete antibodies (proteins) into the body's fluids to ambush and attack antigens (foreign proteins such as bacteria, viruses or fungi). T cells directly attack and destroy infected or malignant cells in the body.

There are two types of T cells: helper T cells and killer T cells. Helper T cells recognize the antigen and activate the killer T cells. Killer T cells then destroy the antigen.

When HIV is introduced into the body, this virus is too strong for the helper T cells and killer T cells. The virus then invades these cells and starts to reproduce itself, thereby not only killing the CD4 T cells, but also spreading to infect otherwise healthy cells.

The HIV virus cannot be destroyed and lives in the body undetected for months or years before any sign of illness appears.

Gradually, over many years or even decades, as the T cells become progressively destroyed or inactivated, other viruses, parasites or cancer cells (called "opportunistic diseases") which would not have been able to get past a healthy body's defense, can multiply within the body without fear of destruction.

Commonly seen opportunistic diseases in persons with HIV infection include: pneumocystis carinii pneumonia, tuberculosis, candida (yeast) infection of the mouth, throat or vagina, shingles, cytomegalovirus retinitis