There's an interesting book called 1491, about the European takeover of America. It has a lot of fascinating stuff about why the American Indians fell like straw before the viruses brought by Europeans.

The author dug up descriptions from the 15th century of beaches piled with bodies, and uses archeological evidence to say that the American Indians were not the hunter-gatherers of the 19th century at the time of the invasion, but were a huge population of city-dwellers, who fell to a gigantic pandemic, partly because their genetic structure was different enough to stop them fighting the infection.

And the really scary thing? It was started by a guy called Bernard (?) de Soto, who brought five pigs in. Here's an article by him about the same subject.