Syphillis is believed to have moved from the Old World to the New around the time of Columbus. The first recorded outbreak is believed to have come from Naples after it fell from the forces of Charles the VIII and it spread among his mercenary army. When the army disbanded they spread it throughout Europe and syphillis began to spread around the continent. It is possible that it developed independently on the two continents and there is evidence to support this in graveyards on both continents. It is possible that Columbus’s crew contracted it in the Dominican Republic and began spreading it among other populations that previously had no contact.
Originally physicians seemed to believe it was just a slight variation of leprosy and they treated it with mercury. It is possible that there was a human-simian jump at some point and there are two strains in Guyana that are very similar to the venereal syphillis that we see today. Those strains are likely to have evolved in those regions to make themselves more effective at transmitting themselves.
I think the pathogen became more like it is today as people became more promiscuous because the disease understood in it’s own way that if it wanted to infect as many people as it possibly could. With this realization probably came the development that it could infect through sexual fluids and be more effective.
It originally manifested itself in bone issues and people thought it was a vitamin A overdose so it caused things like calcification in the tendons which does not seem to appear now it manifests more in the chancres, the open sores on the body, on the genitals so it became more virulent with time.