As a result of the interactions between the Old World and New World that began with Christopher Columbus' journey to the Americas, several new diseases were introduced to the Native American population including influenza, smallpox, malaria, measles and yellow fever. The Native Americans' immune systems, having never been exposed to the diseases prior to the arrival of the Europeans, proved highly susceptible to the new pathogens. Some estimates place the number of deaths from European diseases among Native American populations as high as 80 to 95 percent during the first 150 years following Columbus' 1492 landing.
Most of the diseases brought to the Americas by Europeans were highly contagious and spread rapidly among the Native American population, even in cases in which the affected groups had no direct contact with Europeans. Smallpox, a disease for which a significant number of Europeans had already developed an acquired immunity, was the deadliest of the new diseases brought to the American shores. A comparatively much greater loss of life from smallpox occurred among Native Americans than the fatalities suffered by Europe during the widespread bubonic plague, known as the Black Death.
Yellow fever on the American continent is believed to have been a result of the Atlantic slave trade. The slaves that were brought to the Americas had already developed an acquired immunity to the disease in Africa, but the Native Americans and European settlers' immune systems were unprepared to fight the disease and yellow fever epidemics continued to break out in the colonies up until the 1800s.