Agricultural societies are sedentary and able to support larger, denser populations. Naturally, these dense surroundings make for an environment in which infectious diseases spread more quickly. Those diseases that have evolved to spread quickly through a population rather than what Diamond calls a "steady trickle of cases" also tend to leave people immune to another case. So they develop in populations that are dense enough to present a new group of potential victims within a few years. Another major factor contributing to the rise of these diseases among agricultural societies is that many of them originate with animals, especially the types of domesticated animals that are used for food, labor, and companionship in agricultural societies. Also, farmers, as Diamond points out:
are sedentary and live amid their own sewage, thus providing microbes with a short path from one person's body into another's drinking water.
Hunter-gatherer societies move around frequently enough that this is not a sustained problem.
Source: Diamond, Guns, Germs, and Steel, 205-210.