Final answer:
HIV progresses to AIDS when the CD4 T-cell count drops below 200 and the patient has at least one AIDS defining illness. These illnesses often indicate a severely compromised immune system.
Step-by-step explanation:
HIV is considered AIDS once the CD4 T-cell count is <200 and a patient has at least one AIDS defining illnesses. Acquired immunodeficiency syndrome (AIDS) is the late stage of the human immunodeficiency virus (HIV) that severely damages the body's immune system. AIDS is diagnosed when the helper T cell count falls below 200 helper T cells per microliter of blood or when the individual develops certain opportunistic diseases. These diseases, which the immune system of a healthy person would typically resist, include rare cancers like Kaposi's sarcoma and infections such as Pneumocystis pneumonia and toxoplasmosis. Without treatment, the progression of AIDS can lead to a fatal outcome.