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 whether syphilis, for example, reached Europe from America).
The fundamental reason for this asymmetry, however, resides in a factor that apparently could seem natural, ecological or even flower power : animals .
Domestication and zoonotic diseases
Most Old World diseases originated in animal reserves, especially domesticated animal farms, which were not present in America .
Native Americans had hardly any domesticated farm animals available, and therefore there were not many zoonotic diseases (the kind that are 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 came 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 the newly arrived Europeans.
The exchange between the Old and New World was very fruitful in terms of agricultural products: America supplied Europe with corn, potatoes, and tomatoes; Europe procured America for wheat and rice. Sheep, goats and pigs also came there. And addictive products also flowed bi-directionally: tobacco or sugar cane. But diseases were much more prevalent in the New World simply because the natives were not so used to domesticated animals .