Ok, maybe i can explain, a working girl, well, its their job, its their income. If they are infected with something and word gets around, thats it, no money. So firstly, its in their own interests to ensure they are clean. Secondly, believe it or not, they dont want to be infected with something either, so they are also on the look out for strange symptoms etc. Thirdly, quite a few working girls are clued up on STD's. Fourthly, when they go to the clinic for their regular check up, the clinic staff will talk to them about the risks, symptoms etc.
A civvie is more likely to sleep around without condoms, after all, its just a few men right, probably more likely on a night out to be pissed and more likely to have unprotected sex. They are also less likely to go to the clinic for regular check ups and less likely to spot symptoms (or lack of them) if something did crop up.