The human immunodeficiency virus (HIV) is a lentivirus that attacks the immune system. Because it has a long incubation period, it may be a decade before an infected person shows any symptoms. HIV is the virus that causes acquired immune deficiency syndrome (AIDS), and, because of its destruction of the immune system, it frequently also leads to other opportunistic infections. Although there are recorded instances of single cases of infection reported in the mid-20th century, the first case of today's HIV/AIDS epidemic was reported in 1981. By 2014 more than 35 million people were living with HIV worldwide [source: amfAR].
While we still can't be positive, scientists have a leading theory of HIV's origins. According to the "hunter" theory, Simian Immunodeficiency Virus (SIV), the type that infects primates, jumped from primates to humans through the butchering and eating bushmeat; this strain would become known as HIV-1. Other theories remain controversial or have been debunked, including the idea that the virus was transferred through the oral polio vaccine or through sharing contaminated needles during inoculations. It's also been suggested that HIV is a genetically modified organism and that it's a government conspiracy against black and gay populations.
Not only did we not know the origin of the virus early on, we didn't even know how it spread. Some thought HIV spread just by touching an infected person, or by touching an object after an HIV-positive person had. That's incorrect. You're not at risk through handshakes, hugs, kisses, sharing a toilet, using the same exercise equipment or touching a doorknob. Some thought HIV spread through saliva, sweat and tears — it doesn't. In the early '80s, a Canadian flight attendant named Gaetan Dugas was fingered as "Patient Zero" of the HIV epidemic because of his extensive travels and HIV-positive status. But he wasn't; the HIV entered the U.S. long before his career in travel began. Nor is there any evidence the virus is passed through mosquito bites. And since 1985 the Public Health Service has been screening donated blood for HIV. And no, HIV is not airborne.
Despite the myths about HIV transmission, the truth is that the virus spreads only through contact with certain HIV-infected bodily fluids, including breast milk, blood, semen and vaginal fluid, and often through unsafe sex or through sharing contaminated needles.