Contact with Europeans exposed Native Americans to diseases against which they had not developed resistance -- small pox, measles, chickenpox, influenza, malaria, and yellow fever. Economically, the population decrease brought by the Columbian Exchange (decimation of Native American populations by disease) indirectly caused a drastic labor shortage throughout the Americas, which eventually resulted in the establishment of African slavery on a vast scale in the Americas. Europeans, who settled in the New World, also chose to cultivate Old World crops, such as wheat and apples. Europeans in sowing these crops also inadvertently introduced various Old World weeds. The New World's greatest contribution to the Old was in crop plants. Maize, white
