Before there was regular communication between the two hemispheres , the different types of domesticated animals and diseases were much larger in the Old World than in the New .
Before regular communication had been established between the two hemispheres , the varieties of domesticated animals and infectious diseases , such as smallpox , were strikingly larger in the Old World than in the New , in part because many migrated west or were brought by traders from Asia , so diseases of two continents were suffered by all .