<P> European exploration of tropical areas was aided by the New World discovery of quinine, the first effective treatment for malaria . Europeans suffered from this disease, but some indigenous populations had developed at least partial resistance to it . In Africa, resistance to malaria has been associated with other genetic changes among sub-Saharan Africans and their descendants, which can cause sickle - cell disease . </P> <P> Before regular communication had been established between the two hemispheres, the varieties of domesticated animals and infectious diseases that jumped to humans, such as smallpox, were substantially more numerous in the Old World than in the New due to more extensive long - distance trade networks . Many had migrated west across Eurasia with animals or people, or were brought by traders from Asia, so diseases of two continents were suffered by all occupants . While Europeans and Asians were affected by the Eurasian diseases, their endemic status in those continents over centuries resulted in many people gaining acquired immunity . </P> <P> By contrast, "Old World" diseases had a devastating effect when introduced to Native American populations via European carriers, as the people in the Americas had no natural immunity to the new diseases . Measles caused many deaths . The smallpox epidemics are believed to have caused the largest death tolls among Native Americans, surpassing any wars and far exceeding the comparative loss of life in Europe due to the Black Death . It is estimated that upwards of 80--95 percent of the Native American population died in these epidemics within the first 100--150 years following 1492 . Many regions in the Americas lost 100% . The beginning of demographic collapse on the North American continent has typically been attributed to the spread of a well - documented smallpox epidemic from Hispaniola in December 1518 . At that point in time, approximately only 10,000 indigenous people were still alive in Hispaniola . </P> <P> Similarly, yellow fever is thought to have been brought to the Americas from Africa via the Atlantic slave trade . Because it was endemic in Africa, many people there had acquired immunity . Europeans suffered higher rates of death than did African - descended persons when exposed to yellow fever in Africa and the Americas, where numerous epidemics swept the colonies beginning in the 17th century and continuing into the late 19th century . The disease caused widespread fatalities in the Caribbean during the heyday of slave - based sugar plantation . The replacement of native forests by sugar plantations and factories facilitated its spread in the tropical area by reducing the number of potential natural predators . Yet, the means of the transmission was unknown until 1881, when Carlos Finlay suggested that the disease was transmitted through mosquitoes, now known to be female mosquitoes of the species Aedes aegypti . </P>

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