Fun tidbit:
> But contrary to popular belief, it wasn’t all one sided. It’s believed that one Native American disease did slip on to the European ships and sailed onward to Europe doing some major damage in the process. That disease was syphilis.
Infectious diseases often arise when humans live in large groups with their animals from which a lot of the diseases originate.
Another uneducated guess is that this is why your pet <insert species name here> doesn't often get sick - they have much less contact with other <insert species name here>s than you do with other humans, contributing to less and slower spread of disease.
Please could a pathologist jump in and enlighten me?
Not a bad guess, but completely wrong. The cities of Mesoamerica or the Andes or the Mississippian culture of North America would have dwarfed most European cities, before they were ravaged by disease.
The best explanation I have seen is that almost all domesticable animals in the New World were exterminated early, and so there wasn't anywhere near the cross-species disease potential in the Americas. Old Worlders had domesticated horses, goats, sheep, pigs, cows, a wide array of fowl, camels, dogs, cats, etc, etc. In the Americas, there were turkeys, ducks, llamas & alpacas, and guinea pigs - and not all of those in any one place.
East-west contact within Eurasia of course did bring some serious diseases to the West, to which people had no resistance. Plague in the 14thC, cholera in the 19th. (Did any similar diseases arrive in the East? I can't think of any.)
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.