How does deadly epidemics actually help certain societes?
In the context of Diamond's book, one could argue that falling victim to deadly epidemics helped societies by giving the survivors immunity to the diseases that caused the epidemic. When people from European societies encountered people in the New World, they brought the germs that caused epidemics with them. Native Americans had no immunity to these diseases, so they died in droves. As Diamond shows in his chapter on Pizarro's conquest of the Inca, these dreadful outbreaks, along with the technology they brought with them, made it far easier for Europeans to conquer Native American peoples. This, of course, would be small consolation to the people who had perished from the frequent epidemics in Europe. A better way to think about it, perhaps, is that the fact that the conquerors came from societies that frequently experienced epidemics made their conquests easier to achieve. It is difficult to argue that these epidemics helped their society at large.