The section of this magnificent work you want to examine is Chapter Eleven, entitled "The Lethal Gift of Livestock." In this chapter, the author explores how the domestication of animals by some societies resulted in their exposure to a greater number of germs and bacteria, allowing them to develop resistance to such germs that gave them a significant advantage over other societies that did not domesticate animals, and therefore did not have the opportunity to develop resistance to diseases that find their origin in animals.
This chapter explores how the microbes that domesticated animals bombard us with turn into diseases, such as how smallpox emerged from cattle. Initially, such civilisations suffered greatly as such diseases developed and infected humans, but afterwards they gained immunity--an immunity that other civilisations lacked. Note the importance that the author places on such germs in the conflict of the Old and New Worlds:
The main killers were Old World germs to which Indians had never been exposed, and against which they therefore had neither immune nor genetic resistance. smallpox, measles, influenza, and typhus competed for top rank among the killers.
Thus we can see how the author establishes a causal link between owning livestock and therefore gaining immunity to diseases that had such a massive impact when such civilisations met others.