* 1 Infection by HIV * 2 Diagnosis o 2.1 WHO Disease Staging System for HIV Infection and Disease o 2.2 CDC Classification System for HIV Infection o 2.3 HIV test * 3 Symptoms and Complications o 3.1 The major pulmonary illnesses o 3.2 The major gastro-intestinal illnesses o 3.3 The major neurological illnesses o 3.4 The major HIV-associated malignancies o 3.5 Other opportunistic infections * 4 Transmission and prevention o 4.1 Sexual contact o 4.2 Exposure to infected body fluids o 4.3 Mother to Child Transmission (MTCT) * 5 Treatment * 6 Epidemiology * 7 Economic impact * 8 Stigma * 9 Origin of HIV * 10 Alternative theories * 11 Notes and references * 12 External links