Once a month or so, I receive an e-mail from a man who has visited a prostitute for a blow job and wants to know what the risks of oral sex are. These questions are often from men who have visited high-risk prostitutes outside the U.S.; however, that generally only changes the magnitude of the risk, not the nature of it. Unprotected fellatio is potentially risky for any man who receives it, not just for those who visit prostitutes and other commercial sex workers.
Men who visit prostitutes in the developing world and other high-risk areas are often particularly concerned about contracting HIV through oral sex. HIV can be transmitted through oral sex, although the risk of contracting HIV is much higher during other forms of sexual activity. However, since oral sex does include a risk of HIV, it is always a good idea to use a condom during oral sex -- particularly with an untested partner or a partner who is high-risk, such as a prostitute. Condoms are very effective at reducing HIV risk.
HIV is not the only potential negative consequence of receiving a blow job from a prostitute or other partner. Syphilis and gonorrhea can both be transmitted during a blow job, as can HPV and herpes. As with HIV, practicing safe oral sex will reduce your risk -- although condoms may not be 100 percent effective at preventing the diseases that can be spread from skin to skin.
Under no circumstances should you simply wait to see if you develop symptoms to determine whether your sexual encounter has exposed you to an STD. Many men with STDs never show any obvious symptoms, but they can still suffer long-term health effects or pass the infections onto their partners. Fortunately, except for HPV, all of the above diseases can be screened for using blood or urine testing, but it may take several months to be certain that your results are reliable.
A lot of men want me to quantify their level of risk after they've received an unprotected blow job from a prostitute. The problem is that there's no practical way to do so. The most accurate answer I can give is that it is possible that they've been exposed to and infected with one or more of the STDs described above, and the only way to be certain is to get tested. Furthermore, since blood tests that are antibody based can take a few months to produce reliable results, they should consider the possibility that they are putting their sexual partners at risk during future sexual encounters and discuss their risk and practice safe sex accordingly.
Note: Some commercial sex workers are extremely reliable about practicing safer sex and undergoing regular testing -- much more so than many people in the general population who simply have a lot of sexual partners without getting paid. That's why, unless you do your homework, you have to assume every sex partner is potentially a source of risk, whether you're paying them or not.