Looking at the chart in Figure 4.1, we can see that epidemic diseases are the most direct product of agricultural society. This is the proximate factor that has a direct line from a factor that is closest to the ultimate factors.
Epidemic diseases are caused directly by the existence of a farming society. In farming societies, people come into very close contact with animals. This allows diseases to jump from animals to human beings. In those societies, there are relatively large populations. These large populations allow the diseases to have many hosts and, therefore, to stay alive (rather than killing all their hosts and dying themselves). Thus, epidemic diseases are the most direct result (of the proximate factors) of the rise of agricultural societies.