Herpes is probably the biggest STD risk during oral sex. Both strains of herpes can live in the mouth or the genitals, and particularly during outbreaks (cold sores, herpes lesions) can be passed from one place to the other. Many people have oral herpes, more than 50% of a random group of people will have antibodies to the virus (indicating some level of infection). Having oral herpes for most people is no big deal, a cold sore during times of stress or illness is usually little more than an annoyance. Genital herpes can be more complicated and uncomfortable, but there are treatments (see Herpes Fact Sheet for more info). Care should be taken so that oral herpes are not passed to anyone’s genitals. In general it is not a good idea to have unprotected oral sex while any lesions are present.
Chlamydia and gonorrhea can infect your throat, strep like symptoms, and are curable with antibiotics. These can also infect the eye, and though rare, eye infections can have serious consequences, so be careful with fluids.
HIV can be passed through unprotected oral sex, but it is more likely to be passed during unprotected penetrative sex. The infected semen/precum or vaginal fluid must enter the body through a cut or sore in the mouth or esophagus. The virus is unlikely to be passed from a person’s mouth to another person’s genitals.
HPV can be passed during oral sex, but it is rare. HPV has been found on vocal chords.
Syphilis can be passed similar to HIV. Syphilis is curable.
Hepatitis A is a risk particularly in oral-anal sex, and people engaging in this unprotected might want to look into getting a Hep A vaccine. Hep A is not a chronic condition like Hep B and C, but can make a person quite sick several weeks.
To reduce the risk of infection during unprotected oral sex, limit exposure to sexual fluids and ensure that no cuts or lesions are present in mouth or on genitals.
Use barrier methods — though many people feel that barrier methods detract from oral sex, they are very effective at preventing STD transmission:
Oral-penile sex: Male condom, no spermicide (kills taste buds), flavored condoms are available.
Oral-vaginal sex: Dental dams (aka Lollys) — can buy them, or you can use non-microwaveable saran wrap, or cut up a latex glove.
Oral-anal sex: Dental dams, non-microwaveable saran wrap, cut up latex glove
Spit or swallow? Limiting exposure to semen reduces risk of infection, so ejaculation away from partner’s body is the safest. That said, the mouth is the place most likely to contract something like HIV, so the conventional wisdom is, “swallow or spit, just don’t let it sit.”