Comment by larksimian

It's not really random. Disease development in the Old World was a combo of long term urbanisation and animal husbandry. Urbanisation in the New World existed but wasn't as old and obviously there was way less animal husbandry. Also assuming the Bering straight crossing hypothesis humans probably left almost every freaking old world parasite and disease behind by the time they got into the americas. Hundreds of thousands or even millions of years of eveolution of diseases that evolved in Africa/Eurasia pretty much gets purged during that migration.