Human immunodeficiency virus (HIV) is contracted through sexual contact via anal, oral, or vaginal intercourse; blood exchange; shared intravenous syringes; or from mother to child during pregnancy, labor, delivery, or breast-feeding. It originated in primates but ultimately transmitted to humans. The first cases were discovered in Africa in 1950s and 1960s, and the disease spread rapidly throughout the globe, affecting more than 36 million people by 2001.