In 1981, medical literature first described an unusual constellation of symptoms in five young homosexual men in Los Angeles. Subsequent reports described similar symptoms in residents of other cities, such as San Francisco and New York, not only among men who were having sex with men (MSM) but also among individuals who injected drugs and in people with hemophilia. These cases suggested that the cause was in the blood and was transmitted by sexual contact, sharing infected drugs, or receiving blood donations. In September of 1982, the Centers for Disease Control and Prevention (CDC) published a case definition using the current name acquired immune deficiency syndrome (AIDS).

It is now known that AIDS is caused by the human immunodeficiency virus (HIV). The virus can be transmitted ...