How did so many diseases get to America?

The Old World carried large numbers of diseases to the New World, but disease transmission was not bilateral. At least not in the same proportion (it is still debated if syphilis, for example, came from America to Europe).

The fundamental reason for this asymmetry, however, resides in a factor that apparently could seem natural, ecological or even flower power to us: animals.

Most of the diseases of the Old World originated in animal reserves, especially in the farms of domesticated animals, which were not present in America.

Native Americans hardly had domesticated farm animals, and therefore there were not many zoonotic diseases (the kind spread by close contact between animals and humans). As Jeffrey D. Sachs explains in his book The Ages of Globalization:

The list of diseases that arrived from Europe was long and deadly, including smallpox, influenza, typhus, measles, diphtheria, and whooping cough. Smallpox was the great mass murderer: it wiped out an alarming proportion of native populations that encountered newly arrived Europeans.

The exchange between the Old and the New World was very fruitful as far as agricultural products were concerned: America provided Europe with corn, potatoes, and tomatoes; Europe provided America with wheat and rice. Sheep, goats and pigs also arrived there. And the addictive products also flowed bidirectionally: tobacco or sugar cane. But disease was much more rampant in the New World simply because the natives were not as accustomed to domesticated animals.

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