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It is well known that the European colonists brought many infectious diseases to the Americas, and that these had a deadly effect on the native populations, because they had no immunity to them. Were there any local infectious diseases to which the colonists were not immune? I’ve never heard of such. I’m not aware that the colonists suffered any epidemics, or that they brought any new and unusual diseases back to Europe.

Why not? Is this merely an accident of history, that there were no infectious diseases in the Americas which did not already exist in Eurasia? Or is there some explanation?

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I assume because if a disease is contracted and mutated at a certain rate, then the chances of it developing further into new strains in a large number of population is faster than lets say the few native colonies, hence if you stretch this through time you can see that even the simplest of viruses that people have build immunity to over say 100 years can be deadly to a few that might have never encountered the virus before. but thats just my guess. –  Bez Aug 23 at 21:22
    
    
Malaria killed a large number of English colonists, e.g., at Jamestown. That wasn't west-to-east transmission, but it was transmission from Native Americans to Europeans. Malaria is a tropical disease, so it wasn't going to be transmitted to England. More generally, the people who came across the Bering Land Bridge went through a kind of quarantine, so you'd expect a certain number of infectious organisms to die out. –  Ben Crowell Aug 24 at 23:28
    
@BenCrowell That malaria may have been carried by the slaves instead of being indigenous in the natives. en.wikipedia.org/wiki/History_of_malaria#Spread_to_the_Americas –  user137 Aug 25 at 2:04

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In "Guns, Germs, and Steel" Jared Diamond includes quite a bit on this topic. His conclusion is that Europeans, and old world humans in general were much more exposed to their farm animals, often living in the same buildings. This allowed a much greater number of diseases to jump from animal to human, forcing us to development immunity against these pathogens.

The native americans never domesticated as many animals, and weren't exposed to as many pathogens. As a result the foreign pathogens could freely move through their populations.

So why did it only go one way? A lot more Europeans came to American than vice-versa, so there just wasn't as much opportunity for American pathogens to move to Europe. Additionally, the Europeans brought a lot of animals here, including cattle, horses, and pigs. These would have carried pathogens as well, again, very few American animals were taken to Europe.

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I find this very hard to believe (though I know absolutely nothing about it so I am likely wrong). Surely at least the parasites were a problem for the colonists! We know there are all sorts of parasitic pathogens endemic to the Americas, none of which would be familiar to the European immune systems. Weren't they an issue? –  terdon Aug 23 at 22:37
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I'm not an expert on parasitic diseases, but off the top of my head parasites just don't seem as contagious or as deadly as the viral or bacterial diseases of the time. Some parasitic diseases did slow European colonization of Africa, particularly malaria and sleeping sickness, but these weren't present in the Americas at the time. And the same conditions that exposed Europeans to more viruses and bacteria would have probably exposed them to many common parasites, possibly conferring at least some resistance to the parasites they encountered here. –  user137 Aug 24 at 3:13
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@user137 - As a small extension of what you've written, I'd suggest pointing out that most Europeans who came over here and got sick probably died here rather than take the disease back to Europe. –  Bobson Aug 25 at 2:04
    
Yeah, we don't remember how many people were killed by disease. Cities were so bad that deaths outpaced births until the 19th century, they relied on immigration to sustain themselves. –  user137 Aug 25 at 2:22

There is at least one important exception - it is generally thought that syphilis came to Europe from the Americas.

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But as far as we know it's the only exception. And the percent of Europeans killed by it was much smaller than the percent of Americans killed by European pathogens. And the 2011 paper that supports an American origin for syphilis analyzed skeletal evidence. I'd like to see PCR evidence for the bacteria in skeletons dating prior to 1492 on either side of the Atlantic, but 600 year old skeletons probably aren't the easiest to get treponema DNA from. –  user137 Aug 23 at 21:39
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@user137: There's also direct genetic evidence from the bacteria itself the that modern syphilis descends from a related South American disease. –  slebetman Aug 24 at 0:32
    
@slebetman, thanks, that sounds like better evidence. –  user137 Aug 24 at 0:47
    
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@user137 : this paper presents interesting phylogenetics for Treponematoses which suggests the most recent ancestor of venereal syphilis was a central American variety of Yaws (caused by a very similar organism). plosntds.org/article/info:doi/10.1371/journal.pntd.0000148 –  Dan Sheppard Aug 25 at 0:53

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