You can answer this question by reading in Chapter 11. There, Diamond tells us that civilization leads to epidemic diseases because it creates large pools of hosts for diseases and it puts them in contact with animals.
Civilization starts with farming and farming typically involves close contact between people and animals. This allows germs to jump from animals to people. Table 11.1 shows a number of human epidemic diseases that originated in animals. Civilization leads to high population densities. High population densities, in turn, give diseases enough hosts to survive. In smaller populations, epidemic diseases kill all their hosts and die out themselves. In large populations, epidemic diseases can survive and become chronic parts of our lives, causing periodic epidemics while still leaving enough people alive to continue to host the diseases.