[jaguar]

Silent is on the right track, you can't stop prostitution, making it illegal just makes it harder for them to get help and keep safe. At least with regular screen, treatment and promotion of the use of protection you can minimise the damage rather than moralising and pretending it doesn't happen. It doesn't make the problem go away but it's better than nothing. Very detailed writeup here. To quote:

With sexually transmitted diseases (STDs) like syphilis, gonorrhea, chlamydia and herpes, prostitutes must be monitored to prevent the spread of these afflictions. Chancroid, a STD typically found in third world nations, is occurring in places throughout the U.S. due to transmission brought on through illegal prostitution (Schmid, Sanders, Blount & Alexander, 1987). Chancroid makes ulcers in the vagina that assist with the spread of HIV/AIDS.

A Public Health Review of Chancroid from the World Health Organization stated:

In Kenya, where the importance of chancroid in HIV transmission was first described in the late 1980s, interventions targeting sex workers and STD patients were implemented. Reported condom use by sex workers has since increased to over 80% in project areas and the incidence of genital ulcers has declined. Chancroid, once the most common ulcer etiology, now accounts for fewer than 10% of genital ulcers seen in clinics in Nairobi, Kenya.

In Senegal, HIV prevalence among pregnant women has been below 1% for more than a decade. A strong multisectoral response, an effective STD control programme and early legalization of prostitution have been credited for this low level. Special clinical services, for example, offer regular examination and treatment for registered sex workers. Not only has there been a significant decline in STD rates among sex workers and pregnant women between 1991 and 1996, but genital ulcers are also no longer common and chancroid is reportedly rare. (Steen, 2001)