The answer to this can be found in Chapter 11. One place to find it is in Table 11.1.
The basic answer to this is that people get diseases from their domesticated animals. In early farming societies, people live in very close proximity to their animals. They will have animals in their houses with them. They will often come in contact with body fluids from animals as they milk them or kill them. Because of this, germs can pass very easily from domesticated animals to human beings. A list of the diseases (many of which are or used to be fatal to people) that have come from domesticated animals in found in Table 11.1.
Thus, the gift of livestock can be lethal because germs from animals can pass to people and mutate into epidemic diseases that kill people.