When an HIV positive individual's T-cell count falls below 200 cells per cubic millimeter, he has progressed to stage 3 HIV and has AIDS, advises AIDS.gov. HIV positive individuals are also diagnosed with AIDS if they also have at least one opportunistic illness.
T-cells, or CD4 cells, are a type of white cell that fight infection. The level of these cells in the blood indicate how well a person's body is able to fight the HIV virus and other illnesses, states AIDS.gov. In a health person, the T-4 cell count ranges between 500 and 1,600 cells per cubic millimeter. The HIV virus attacks T-cells and uses them to replicate itself.
A blood test for antibodies is the most common way HIV is diagnosed, states the UCSF Medical Center. After positive diagnosis, patients should have their T-cell count measured and monitored every three to six months to track the progression of the disease and adjust treatments, states AIDS.gov.
Antiretroviral therapy, or ART, can help slow the progression of HIV, but cannot cure the disease, advises AIDS.gov. Once a person is diagnosed with AIDS, he lives an average of three years without further treatment. If he contracts an opportunistic infection, his life-expectancy shortens to approximately one year. Some common opportunistic infections include invasive cervical cancer, herpes simplex, bronchitis, pneumonia and tuberculosis, according to the CDC.