Diseases that are transmissible via unprotected oral sex include herpes, gonorrhoea, syphilis and hepatitis. A low risk also exits for contracting HIV during oral sex.
Herpes can be easily transmitted from genitals to mouth, or mouth to genitals during unprotected oral sex. There are two varieties of the herpes simplex virus – Type I typically infects the facial region and Type II the genital region.
Herpes can also be spread through contact with an infected person who has no symptoms, but some people feel the risk is acceptably low outside of the most infectious period (which starts with the tingling sensations that precede an outbreak, and continues for two weeks after the herpes sores go away).
You can, on rare occasions, contract a bacterial infection by performing oral sex on someone who has a bacterial STD (typically gonorrhoea, more rarely syphilis or chancroid), but these can usually be cured with antibiotics once they're diagnosed.
The risk of transmitting HIV is much lower for unprotected oral sex than for unprotected anal or vaginal intercourse, and the risk is much lower for the person receiving oral sex than for the person performing it.
For the person performing oral sex, the risk of transmission is lower if your gums, lips, mouth and throat are healthy and contain no lesions, if you don't let men ejaculate in your mouth, and if you don't perform cunnilingus (oral stimulation of the female sex organs) on a woman while she is menstruating.
The HIV virus can be present in male ejaculation and pre-ejaculation. The risk increases for a woman's partner while she is menstruating, since HIV is found in highest concentration in the blood (it can also be found in vaginal and cervical secretions).
Some safe-sex experts recommend not flossing or brushing your teeth for an hour before giving unprotected oral sex, as this may cause slight lesions and bleeding; others recommend visually checking your partner's genitals for signs of contagious STDs (including genital warts, which can on rare occasion be transmitted from genitals to mouth).
If your partner ejaculates in your mouth during unprotected fellatio, you can reduce your risk for STDs by immediately spitting. It may help (especially for bacterial STDs) to then use an anti-bacterial or peroxide mouthwash.
Opinions differ on the importance of safer-sex barriers for oral sex, but many experts recommend using some form of barrier protection (condoms or latex dental dams) if you feel uncertain about whether you or your partner have STDs.
You can also use a femidom (the female condom) or cut a condom open and use it as a barrier between mouth and genitals during cunnilingus or anilingus (oral stimulation of the anal region). Continue to use condoms and dental dams until you have both been tested.
Oral sex can be a lot of fun for both parties, just remember to play safely.