I've heard a lot about smallpox and the other diseases that came to the Americas from Europe. But I don't recall hearing about many diseases going the other direction. Were there any new, major diseases in the Americas that hadn't yet made it to Europe/Asia/Africa?
Chagas disease is a native South American. Although it was only first described by (and named after) Carlos Chagas in 1909, so that probably isn't what you're looking for.
The most prominent one would be syphilis, if we could finally fix the location of its origins properly. It is still somewhat unclear whether Columbus brought it back home to Europe (Columbian hypothesis) or whether it was a mutation of an already existing European strain (pre-Columbian hypothesis).
Bruce Rothschild suggests that the typical bone alterations have been present and become almost identical to today's characteristics all over North America by the time Columbus arrived and is in fact very well-documented in the Dominican Republic, while he can't identify any syphilitic skeletons from pre-Columbian Europe, Africa or Asia. The first documented outbreak of syphilis is in Naples in 1495, so after the return of the explorer fleet.
I'm afraid that's all I really got though.
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Native Americans and (European) Diseases - skip the first few posts regarding Norse contact