As far as I understand, the exact history of syphilis is still unknown, and the idea that it was spread to Europe from the Americas is not completely impossible (though seems less likely today).

As for SIV, I am aware of it, but it is not a serious problem for most simians (though apparently Chimpanzees are being killed by SAIDS in serious numbers) - which is my point. STIs have not been so serious a threat to animals as HIV+AIDS has been to humans in the initial pandemic.