You can find the answer to this question in Chapter 11. There, Diamond gives us a number of reasons why farming civilizations are more likely to produce and sustain infectious diseases than hunter-gatherers are. These reasons include:
- Higher population densities. These give diseases more hosts in which to live and evolve.
- Proximity to animals. Farmers live close to animals, particularly in less-developed farming societies. This allows many opportunities for diseases to spread from animals to people, where they can mutate and become crowd diseases.
- Poorer sanitation. Hunter-gatherers move around but farmers stay in one place. Before people understood that human waste could transmit diseases, this led to poor sanitation and more disease.
- More trade between societies. Farming civilizations can create food surpluses. This allows some people to do things like trading with other groups. Trade routes allow diseases to spread to new populations.