The disease that HIV develops into is called AIDS, which stands for acquired immunodeficiency syndrome. AIDS is the most advanced stage of HIV infection, occurring when the immune system is severely damaged, typically indicated by a very low CD4 cell count (below 200 cells per microliter of blood) or the presence of certain opportunistic infections or specific cancers associated with HIV. Without treatment, almost all people with HIV will eventually progress to AIDS, which leads to a greatly weakened immune system and an increased susceptibility to infections and certain cancers.