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If Wikipedia is to be trusted [1]:

"AIDS was first clinically reported on June 5, 1981, with five cases in the United States"

"Both HIV-1 and HIV-2 are believed to have originated in non-human primates in West-central Africa and were transferred to humans in the early 20th century."

If HIV, a relatively less infectious virus, can make its way across the Atlantic Ocean to land on America shores, I wouldn't be surprised SARS-CoV-2 could do the same. Another explanation could be that SARS-CoV-2 is a lot more common than we think --- many people might have caught it before but it didn't manage to spread, but this time the virus just hit the jackpot.

[1]: https://en.wikipedia.org/wiki/HIV/AIDS#History