system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why did HIV appear from nowhere after not existing for millennia?
gpt
We think that HIV-1 (what causes the vast majority of HIV/AIDS cases worldwide) first jumped to humans in 1908, give or take 10 years. We know this because we can monitor the rate at which a virus has mutated against a sample of a known 'control' age. Sort of like looking at before and after photos of a person, you get a sense of the age of the virus.


Prior to 1908 we're certain that HIV existed in primates in localised areas of Africa. How do we know this? We've found same-same but different strains that strongly resemble older HIV virus samples more than the current. Now keep in mind that HIV mutates and evolves at a rate 1 million times that of a human being - beyond a hundred or so years, that virus has mutated so much that it barely resembles HIV at all - only a few key genetic sequences common to every single living (and in viruses case, non-living) thing in existence. So based off this it arose fairly recently in human history.


Why now? Many reasons. Patient Zero probably had contact with a primate through blood entering a cut. Think, a butcher or a trapper. HIV, for all it's virulence, is extremely weak. There have been no recorded cases of HIV transmission through oral exposure. Exclusively sexual contact and blood contamination - environmental exposure kills it fairly quickly. So in the village, it probably spread through sexual transmission, and maybe others got infected too by primate exposure - but this proto-HIV stayed pretty isolated.


But what happened in the early 20th century? Globalisation. Our ability to inexpensively navigate the globe EXPLODED with the advent of air travel, more efficient ships and better combustion engines in land vehicles. Troops began mobilizing in the northern tip of Africa, and subsequent to the first and second wars African colonies were handed over and many colonists decided to return back to their European homelands.

We're talking about the huge relocations of people on a global scale as travel and trade routes spanned the globe. All of this allowed for the early, and relatively harmless, HIV strain to globe trot in the second half of the 20th century.


And from there, somewhere along the way - that virus become HIV-1 Group M and tore apart some poor individual's immune system. The modern iteration of HIV/AIDS came into existence.


**EDIT** There's no reason why HIV arises now rather than say, 100BC. Nature is random and just doesn't give two shits. We just exacerbated it's impact.