Despite what some preachers said at the time, AIDS didn't drop out of the sky to plague the sinful or humble the proud. Instead, it was caused by a virus that simply did what viruses do. (Refer to the preceding section if you're unfamiliar with the general behavior of viruses.)
One thing that's so interesting about HIV viruses is that they're very similar to viruses that infect primates. Until this simian virus infected some unlucky person, AIDS didn't exist in the human population. Once in the human population, the previously simian virus was exposed to a new selective environment — namely us.
We don't know how often such cross-species transfers happen, but we do know (unfortunately) that sometimes the introduced virus heads down an evolutionary path that results in viruses that eat the new host for breakfast. In the case of AIDS, the primate virus can sometimes get a toehold in a human, and then selection sorts through all the various viral mutants, favoring those that are even better at infecting humans. Before such an event happened, AIDS wasn't a human epidemic; now it is.
Another interesting thing is that the primate-to-human transmission of the virus may have happened a whole lot earlier than people tend to think, given that most people think of AIDS and HIV as being "born" in the last decades of the 20th century. One of the reasons we noticed AIDS later may have been that people move around more nowadays. In the old days, you had to walk to the next village to spread whatever germs you might be carrying, and if people occasionally fell sick here and there due to an odd illness, not many people beyond their doctors or families knew about it. But today, with airplanes that can move people and viruses long distances in little time, a disease can get all the way around the world in a day, and many more people can be exposed in a very short period of time.
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