The mutation is thought to have occurred because of the European life style versus the South American life style.
That is, heavy, rarely washed clothes versus rarely clothed natives prancing around the "rain forests".
The original bacterial infection sores, covered by clothes, and not exposed to UV, O2, etc, eventually mutated in to "syphilis".
It wasn't until earlier this decade that the last known infections of the original bacteria were wiped out in the Western Hemisphere.
Thanks for the history. Very interesting.