How do sex workers prevent themselves from transmitting sexually acquired diseases?

Condom vs no Condom: A condom is the most reliable way to prevent most stds. Contact of genital skin to genital skin is the most common pathway for stds to be transmitted; however, hpv/hsv (warts/herpes) can be transmitted from areas that a condom doesn't protect (base of the penis, scrotum). If by sex workers you mean prostitutes, that is the answer. If you mean porn actors, they are all routinely screened for stds.