The answer to this can be found in Chapter 11. In fact, it is hinted at in the title of this chapter, “Lethal Gift of Livestock.” The major effect that domesticated animals had on human beings, outside of the effect that they had on food, was to bring infectious diseases to the human populations.
Diamond argues that human beings would not have had so many epidemic diseases if they had not lived in close proximity to animals. In Table 11.1, for example, he lists a number of human diseases that came to us through contact with animals. He also says, on p. 206 of the paperback edition of the book, that
...when we domesticated social animals, such as cows and pigs, they were already afflicted by epidemic diseases just waiting to be transferred to us.
To Diamond, this is a very important factor in determining how history played out. The people, such as the Europeans, who had been in contact with domesticated animals for a long time had epidemic diseases. The people, such as the Native Americans, who had not been in contact with animals had no epidemic diseases and no resistance to such diseases. This was a major reason why the Native Americans died out in such large numbers when exposed to Europeans.