I've thought about your wish to create a disease-immune America, and I've got to say, I don't see any way it can realisitcally be done. We had Smallpox for 10 000 years, and we're not immune. The Plague swept through Europe over and over again over centuries, and even the hardest-hit areas have an immunity rate of 20 % today.
To be honest, I think your best bet here may be to make it an ASB. Just handwave in the immunities.
Toby the ASB passes near earth, and sees that the Old and New worlds are about to make contact. Being a moral ASB, he can't sit and watch the inevitable megadeath, so he makes the Americas immune to old world disease, and for fairness makes the old world immune to syphilis.
To be honest, I think your best bet here may be to make it an ASB. Just handwave in the immunities.
Toby the ASB passes near earth, and sees that the Old and New worlds are about to make contact. Being a moral ASB, he can't sit and watch the inevitable megadeath, so he makes the Americas immune to old world disease, and for fairness makes the old world immune to syphilis.