Seems to be working just fine...
Syphilis does better, as a disease, than Ebola for the same reasons you win at Pandemic-type games - the slow progression, the low-profile.
Ebola doesn't spread nearly as much, because it's non-airborne and rapidly fatal to a large number of people who contract it. This is why it stays confined to the butt-end of civilization.
Syphilis does more harm overall because it has numbers in it's favour.
People tend to focus more on Ebola because of the high mortaility rate. It has a couple of pretty horrible "What if?"s - principally, what if it goes airborne? I'm not sure a virus with such a high mortaility rate that's been around so long would actually ever go airborne though - from an evolutionary perspective it's a terrible combination.
A virus with high mortaility and rapid spread will rapidly kill all susceptible individuals within it's catchment area, so it's likely that such things have never really gotten off the evolutionary drawing board. The last thing that came close was the Spanish Flu, which was a more fatal mutation of a fairly innocuous airborne pathogen, rather than a more mobile mutation of something unpleasantly fatal like Ebola.
Of course, the above is true of a pre-air-travel world, because rapid spread would kill off everything in the travel radius - because the travel radius was dictated by walking pace, or driving pace... or the speed of ocean liners. In this day and age, it would be much easier for such a thing to have a serious impact.