Can one get an STI from safer sex with a sex worker?

Dear Concerned,

Sex workers are potentially at risk of contracting sexually transmitted infections (STIs) due to increased numbers of partners and incidences of contact. Your personal risk following sexual contact with a sex worker depends on the unknown health status of that individual, similar to the unknown health status of any potential sexual partner.

Having "safe sex" suggests use of a condom during sex. However, "safe sex" implies zero risk of STI transmission and pregnancy; ideally, sex that does not involve the exchange of blood, semen, or vaginal fluids, nor the transmission between partners of the organisms (bacteria, viruses, protozoa, etc.) that cause STIs. However, protected sex cannot guarantee 100 percent effectiveness against STIs and pregnancy.  The term "safer sex," then, more accurately accounts for the risks associated with having sex, even when protected. Regardless of how easy it is to catch an STI, the risk of transmission will be significantly lower if you follow safer sex guidelines, such as always using condoms and dams, with all partners.

The ease of becoming infected with a sexually transmitted organism depends on many factors, including the presence of infection in one's partner, the type of infection, the presence of symptoms, and the type of sex you are having. In order to accurately answer your question, let's start with some basic information about how STIs are transmitted. 

STIs can be transmitted in two ways: first, HIV, chlamydia, gonorrhea, hepatitis B, and other fluid borne STIs can be passed on from one person to another when an infected bodily fluid — blood, semen, pre-cum or pre-ejaculate, cervical/vaginal secretions — comes into contact with an uninfected person's mucous membranes — the lining of the vagina, rectum, urethra, mouth, or throat. Transmission of this kind of STI is most common during vaginal or anal sex. Using a condom with lubricant during anal and vaginal sex nearly eliminates this risk. Transmission is possible but not as common for oral sex givers. Getting as little fluid in one's mouth as possible greatly reduces any probable risk. Using dry condoms and dams for oral sex eliminates the risk of contracting these kinds of infections.

The second type of STI transmission is through contact with an infected skin lesion, such as a sore or wart.  Herpes and HPV (the virus that can cause genital warts — called human papillomavirus) are transmitted in this way. Herpes causes the formation of a fluid-filled blister, which ruptures, forming a sore.  Within seven to ten days, a dry scab forms and heals, leaving no trace. The highest concentration of viral particles is found in the fluid of the blister; therefore, that stage is the most contagious. Skin-to-skin contact, as in genital-to-genital, mouth-to-genital, or mouth-to-mouth contact, between a sore and a partner's uninfected mucous membrane is necessary for transmission. Although it's less likely, it's still possible for someone to transmit herpes without any sores. Transmission can occur during anal, vaginal, and oral sex. If herpes blisters are present on a partner's mouth, it's possible for that person to transmit the virus to an uninfected partner's genitals during oral sex. Similarly, herpes can be transmitted from a person's genitals to an uninfected partner's mouth.

The best way to prevent transmission of herpes is to avoid touching the skin that is impacted, from the time when hypersensitivity of the skin precedes blister formation until the scabs have completely healed. Between outbreaks, condoms and dams significantly lower the risk of transmission during vaginal, anal, and oral sex. However, condoms only cover the penile shaft and the herpes virus can be in an area of the genitals not covered by a condom.

Again, HPV is transmitted by direct genital-to-genital contact. There are about 100 different strains of the human papillomavirus, and typically those that infect the genital region do not infect other areas. HPV can be quite easy to get. Some studies suggest that as many as three-quarters of adults in the United States have been infected with at least one type of HPV. Though viral infections may be present on areas not covered by condoms and dams, using these barriers can greatly reduce the risk of transmission during vaginal, anal, and oral sex.

So as you can see, there is no short answer to your question. It all comes down to activities, levels of protection, and the level of risk a person is willing to take.  To learn more about safer sex, HPV, and herpes, you can read the related questions.