This is why exactly you want to legalize it. Prostitutes tend to accumulate stds which they pass to their clients, which then pass those to other prostitutes. Upon legalization, they would be asked to take tests and those that failed would not be allowed to work so you basically start with a pool of healthy prostitutes.

On client side, they can also have the clients first present papers before availing service. Even if this is not strictly implemented, the diseased clients would be eventually weeded out. If any of the prostitutes caught something from a client, they can eventually narrow down to the possible culprit because of records and have those clients banned.