This was less likely for two reasons. First, the old world had dense concentrations of people for much longer, which domesticated many different diseases, makin

This was less likely for two reasons. First, the old world had dense concentrations of people for much longer, which domesticated many different diseases, making them childhood annoyances. And second, we spent much longer living in Africa, and co-evolved with diseases like malaria, which but left these behind when emigrating via Alaska. (Edit: I forgot syphilis, which is a likely candidate for a new-world disease. But certainly there were fewer.) East-west contact within Eurasia of course did bring some serious diseases to the West, to which people had no resistance. Plague in the 14thC, cholera in the 19th. (Did any similar diseases arrive in the East? I can't think of any.)