Noticing the connection between cowpox and smallpox helped develop a safer vaccine, but prior to that in China and India they invented variolation (grinding up and applying the scabs to cause a milder infection) centuries before European vaccination experiments: https://en.wikipedia.org/wiki/Variolation

Go back and sink Columbus, Europeans will assume they were correct about the massive ocean to the West being too big and dangerous to sail across and ignore that direction for a while longer. No Conquistadors is a huge win for South and Mesoamerica.

If you really want to reshape the history of the New World you've gotta go back like 12,000 years and convince people to capture and train animals like the camelops and American horse instead of simply hunting them. European diseases were so devastating in large part because New World peoples did not have a comparable livestock culture (that being a major source of disease).

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Seriously, it's not hard to understand which is correct. It's a 5 minute thing to learn, there's no need make yourself look so fucking dumb.