The answer to this can be found in Chapter 11. There, Diamond tells us that epidemic diseases can only come about when there is a dense population living in contact with animals. Through contact with animals (which is much more common in farming societies) we come in contact with their diseases. The diseases can jump to us human beings. In a large population, these diseases can eventually come to be epidemic diseases. This is because they make people very sick for a short time but then leave them immune for life. In a large population, these diseases can spread, killing many but not all. Then they lie dormant for a while until a new epidemic when a new generation arises that has not been exposed to them. Such large populations are only possible in civilizations.