Before regular communication had been established between the two hemispheres, the varieties of infectious diseases that spread to humans, such as smallpox, were substantially more numerous in the Old World than in the New. The geography enabled extensive travel and trade between the East and West. Many diseases had migrated west across Eurasia with animals or people, or were brought by traders from Asia. While Europeans and Asians were affected by the Eurasian diseases, their endemic status in those continents over centuries resulted in many people gaining some immunity.