For thousands of years, the people of Eurasia lived in close proximity to the largest variety of domesticated mammals in the world – eating, drinking, and breathing in the germs these animals bore.
Over time, animal infections crossed species, evolving into new strains which became deadly to man. Diseases like smallpox, influenza and measles were in fact the deadly inheritance of the Eurasian farming tradition – the product of thousands of years spent farming livestock.
With each epidemic eruption,
some people survived, acquiring antibodies and immunities which they passed on to the next generation.
Over time, the population of Europe gained increased immunity, and the devastating impact of traditional infections decreased.
Yet the people of the New World had no history of prior exposure to these germs. They farmed only one large mammal – the llama – and even this was geographically isolated. The llama was never kept indoors, it wasn't milked and only occasionally eaten – so the people of the New World were not troubled by cross-species viral infection.
When the Europeans arrived, carrying germs which thrived in dense, semi-urban populations, the indigenous people of the Americas were effectively doomed. They had never experienced smallpox, measles or flu before, and the viruses tore through the continent, killing an estimated 90% of Native Americans.
http://www.pbs.org/gunsgermssteel/variables/smallpox.html
