wager that a lot of that crime stems from the fact that prostitution is illegal, not from the prostitution itself. Where I live, prostitution is legal and the red light districts aren't more 'criminal' than any other area (in fact, due to being legal, brothels are more scattered throughout the city instead of being condensed in one disreputable area, they don't need to streetwalk since they can advertise their services in the papers...)
As for spreading STD I bet 'normal' sleeping around is more risky in that regard, since there people (too) often disregard proper protection.
