The publication, in 1981, of a report announcing the occurrence of a cluster of diagnoses of a rare form of pneumonia due to the microorganism Pneumocystis carinii in a group of male homosexuals in San Francisco constitutes the first notice of the advent of a new disease. It was quickly recognized, subsequent to that report, that the occurrence of this then rare infection was secondary to the profound depletion of the immune system caused by the new disease, which soon acquired the name of Acquired Immune Deficiency Syndrome, or more familiarly, AIDS. The causative organism was identified as a retrovirus, named Human Immunodeficiency Virus (HIV), which attacks the T4 lymphocytes of the immune system.