Like in Nevada? Oh wait, it doesn't happen there because the legal prostitutes are regularly tested. Try again.
What in the world are you talking about? Nevada authorities have a huge problem with STDs spread by illegal prostitution. What in the world leads you to believe that the legalization of prostitution eliminates illegal prostitution? Do you really think most prostitutes and their pimps are rational and that they will obey the law (i.e., get health checkups, shots, register with the state etc) if they are given the opportunity to do so?
Keep on dreaming, bucko!