Epidemics

The virulence of pathogens is related to how quickly they can move from one host to another. The more quickly they jump, the more virulent they are.

In the 3rd world, so I am told, the population density is high and the sanitation is low. Why don't they have epidemics? They do have endemic disease -- malaria, TB and so on. They don't have the reruns of the black death. Why not?

I now have a guess at an answer. Having high population densities and poor sanitation doesn't by itself mean that pathogens move quickly from one host to the next. It is the job of the immune system to protect the individual against invasion, with endemic disease, the immune systems have had a chance to develop defenses against the prevailing diseases going around. Some people will get sick, sure, but many will not and the velocity will still be fairly low, low enough to keep the virulence in check.

It follows, then, that if a novel disease, one that immune systems are not familiar with, were to get loose, then an epidemic could follow. This could be caused by evolution, some new disease evolves from the existing ones (e.g. the new flu) or it could be caused by population movement, diseases that are endemic in one area moving to a new area in which they are unfamiliar.

This might be how the plague developed in the 14th century. I read, if I remember correctly, that one of the things that happened in the early 14th century was increased trade between Europe and the far east. It is fairly well agreed that the plague came from the east, perhaps it became an epidemic due to increased population densities (making transmittal easier) and having a novel pathogen introduced from some distance away.