The book I'm reading called "Guns, Germs, and Steel" also brings up the good argument that part of the reason why Europeans were able to devistate the central and south Americans with small pox was in part because of the long history Europeans had in associating with domesticated animals.
Just some food for thought.
BTW: The image was found here. There's some funny punchlines about how to spin the information. But it's took forever to load, so I got the graphic so you wouldn't have to wait that long.