First, Europeans lived in appalling filth. The sanitary details are well documented elsewhere, the most telling point being that cities of Western Europe only gained population by migration, not internal growth, so bad were the epidemics. It was a millennium-long germ warfare lab.

Second, Europeans and Asians lived alongside livestock such as pigs and fowl, and also rats, all notorious breeders of diseases that humans can catch. The interplay of illnesses between humans and animals fed the epidemics.