There were not, because they weren't seperated in the same way. Australians and Americans had lived seperated from Eurasians for tens of thousands of years. On the other hand, Eurasians were tangentially connected by trade networks, through the silk road. They weren't completely isolated from each other. Diseases spread from Europe to East Asia and visa versa through the Middle East, in a way that they didn't spread to America. So Europeans and Asians suffered from largely the same diseases, and thus had developed similar immunities.