Diamond considers the domestication of livestock to be a “lethal gift” because livestock tend to be the source of major infectious diseases. This can be seen most clearly on p. 207 in the paperback edition of the book. There, in Table 11.1, Diamond gives us a list of “Deadly Gifts from Our Animal Friends.” He notes that such diseases as measles, smallpox, and the flu come to us from animals. These diseases are typically most likely to be transmitted from animals to human beings if the humans are in relatively close and constant contact with those animals. Human beings only come into close and constant contact with animals if they have domesticated those animals.
Livestock are a lethal gift. They are a gift in that they help people have more food. They give milk and cheese and meat. They provide manure for fertilizer and they pull plows to help people be able to grow more plant food. At the same time, however, they are the source of many of the most deadly infectious diseases that afflict human beings. Therefore, livestock are both lethal and a gift.