As you might expect from an Army museum and a pathology institute, we have a lot of photographs of syphilis (and yaws, a related disease). Here's the NY Times on the natural history of the disease:
A Great Pox’s Greatest Feat: Staying Alive
By MARLENE ZUK
Published: April 29, 2008
Research indicates that syphilis became less virulent over time, which probably helped it survive.