A disease typically cited as an example of a disease endemic or native to the New World affecting Europeans is syphilis, particularly the venereal or sexually transmitted variety that is the most common form of syphilis in the West. Although the origins of syphilis is heavily debated among scholars, archaeological and [phylogenetic] (http://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0000148) data strongly suggest venereal syphilis emerged in the America prior to Columbus while non-venereal syphilis may have been present in the Old World. Historic data suggests venereal syphilis was brought to Europe by Columbus and crew from the New World who likely encountered the pathogen in what is now the Dominican Republic. Upon arriving back to European, Columbus crew's transmitted the pathogen to other Europeans and this ultimately led to the first well-established historically documented European outbreak of syphilis in 1495. People who contracted venereal syphilis during this 1495 outbreak suffered from symptoms that occurred at an earlier stage and were more acute than the symptoms associated with venereal syphilis today. Knell suggests this "may have been the result of the disease being introduced into a new host population without any resistance mechanisms," which is what you would expect if Europeans had never been exposed to syphilis before. However, as Knell notes, a change in virulence can also explain the shift to the less extreme form of venereal syphilis seen today.

[It should be noted that an alternative model suggest syphilis,including the venereal variety, was present in the Old World prior to Columbus' voyages.] (http://www.sciencedirect.com/science/article/pii/S0738081X10000350) However, the archaeological and historic data supporting this model however is less than equivocal.

Edits: Grammar, spelling, formatting, and elaboration

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