[derrida]

AIDS is the condition (medically defined in terms of t-cell counts) of a highly suppressed immune system that occurs during advanced HIV infection. (HIV replicates by getting engulfed by white cells (t-cells) then (like most virii) using an enzyme to transcribescribe it's own RNA into DNA, which is then used by the cell to create multiple new copies of the virus, which eventually become so numerous that the cell bursts)

AIDS isn't by definition caused by HIV, just by an extremely low t-cell count. Non-HIV AIDS cases are extremely rare to nonexistant, however.