HIV weakens the immune system, which fights infection. The virus attacks white blood cells, which help kill bacteria, viruses, and fungi. After the initial infection with HIV, a person may have no signs of AIDS for months to years, because it takes time for the virus to destroy the white blood cells. When enough of the white blood cells are destroyed, then the immune system becomes deficient: it is unable to protect the body from infection. Those who develop AIDS suffer from severe infections and types of cancer. The infections and cancers are rare in those who have healthy immune systems.

HIV attacks a certain type of white blood cell, called a T lymphocyte. In addition to destroying T cells, HIV prevents the production of new T cells. Without these white blood cells, the immune system function is weakened greatly. The blood level of special T lymphocytes, called CD4 T lymphocytes, is low in those who have AIDS. The CD4 T lymphocyte count is less than 200 in people with AIDS.