Prior to European contact, the absence of what exposed peoples in the Western Hemisphere to fewer infectious diseases?

Question

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Draft animals, such as horses and cattle, were major carriers of infectious diseases in Europe and Asia. The absence of draft animals in the Americas made it difficult for infectious diseases to spread widely among populations prior to European contact. Microbes and insects were still present and could transmit diseases, but the lack of draft animals was a significant factor in the lower incidence of infectious disease in pre-contact Americas.

Prior to European Contact, the Absence of What Exposed Peoples