Very few viruses have killed as many people and changed society as much as HIV. When it first surfaced in Africa, sexually transmitted diseases were silent killers that weakened immune systems. Sufferers who have the HIV virus eventually got AIDS, a full-blown viral meltdown of the immune system.
This disease has killed millions of people, and because it constantly mutates, has yet to have a cure. There has been speculation that HIV was originally created as a biological weapon, however, those claims have never been proven.