The beginning of the AIDS epidemic was a subtle shift in the genetic material of a virus which allowed the virus to become zoonotic, meaning that it could be transferred from animals to people. Over a period of time which probably lasted for decades, the virus was repeatedly transmitted from primates in Africa to people, and eventually it established a foothold in the human population and began spreading itself beyond the boundaries of the African continent. By the time scientists and medical professionals were able to connect all the dots, the AIDS epidemic had spread across much of the world; as of 2007, over 33 million people worldwide were infected with HIV, the virus which develops into AIDS.
There is some dispute as to the start of the AIDS epidemic, and even now, researchers are still putting together the pieces of the puzzle. It is clear that the HIV virus originated in primates living in Western and Central Africa, and that it was probably transmitted to humans in communities where people eat the meat of primates. Some researchers have suggested that the rise of cities in Africa may also have contributed to the spread of AIDS, by concentrating human populations.
From Africa, the AIDS virus jumped to Haiti, and then it began to spread rapidly in a variety of directions. A gay flight attendant sometimes referred to as “Patient Zero” is often vilified for causing the AIDS epidemic, but all evidence suggests that the virus spread simultaneously in several communities; the HIV virus actually has several different groupings or “clades,” illustrating the fact that it wasn't caused by a single individual.
The clades also suggest something rather more interesting, which is that HIV and AIDS are probably much older than people think. The common knowledge around the AIDS epidemic suggests that the virus originated in the 1980s, but in fact it more probably began spreading in the 1930s, growing exponentially until there were enough cases in the 1980s for medical professionals to realize that they had an epidemic. At least one documented AIDS death dates back to the 1950s, supporting this view.
By learning more about the spread of the AIDS epidemic, researchers hope to discover new techniques for treating and perhaps possibly curing or vaccinating against the disease. For example, different clades respond in unique ways to treatment, illustrating the diversity of the HIV and AIDS viruses, and some clades are more fatal than others; less fatal clades are probably older, as their victims would have lived long enough for the disease to spread further.