The reason of AIDS is still unknown. After a quarter of a century we know, AIDS is a cellular (TH1-modulated) immune deficiency or even a complete lack of it. As a consequence, there is no protection against viral diseases (and infections with mycobacteria, like tuberculosis and leprosy) and cancer cells. This may end in death.
From the beginning it was guessed, it could be a viral infection of the immune system. The virus however (a retro virus is assumed) is not identified according to classical microbiological criteria. On the other hand, just as little, it is not proved that the virus does not exist.