[Cass]

The theory I've heard is that to breed a deadly plague, you need to live in close contact with animals. A well-adapted virus won't usually kill its host, so most deadly viruses jump over from the animal hosts they adapted to. Europe has lots of animals that can be domesticated (Chickens, dogs, cats, pigs, cows, etc) while the animals in Africa and the Americas mostly were the kind you can't domesticate. (Apart from Llamas.)

That's why most deadly plagues came from Europe back then, and now tend to come from places where people still live in very close contact with their lifestock or with wild animals.

[christkv]

Most deadly plagues then as now came from the east which had both the human density and practices of domesticated animals while also eating a huge variety of animals that was natural reservoirs of pathogens. Pretty much every plague with the exception of 1918 flu can trace it roots eastwards.