The most common STD transmitted via oral sex is herpes. There are two main types of herpes simplex virus (HSV): HSV type 1 and HSV type 2. HSV type 1 usually causes cold sores around the mouth, while HSV type 2 generally causes genital herpes sores. However, oral sex can sometimes cause HSV type 1 around the mouth to be transmitted to the genital area causing genital sores in the other person. This process can also work in reverse, with HSV-2 transmitted from the genitals to the mouth of the other person during oral sex.

The human papillomavirus that causes genital warts can very occasionally be transmitted through oral sex, causing warts to appear around or inside the mouth in anyone who has given oral sex to an infected person.

Gonorrhea has been shown to infect the throat of some people who have given oral sex to an infected person. This infection can then be passed on from the throat to the genitals of any future partners. The body will almost always naturally clear the throat of the bacteria that cause gonorrhea within three months, although infections in the genital tract will usually require antibiotics to cure. Chlamydia can also infect the throat in a similar way, although this is less common. Both infections may result in a sore throat, although many people will remain asymptomatic and unaware they are infected.

Syphilis may be passed on during oral sex if a person’s mouth comes into contact with an open sore or a skin rash caused by the infection.

Gastrointestinal infections and parasites may be passed on during oral contact with the anus.

The hepatitis A virus is also contained in human feces, and may be passed on during anilingus.

Hepatitis B is contained in sexual fluids and blood and may be transmitted during oral sex in a similar way to HIV (see below).

Hepatitis C is generally only contained in blood, and will only be transmitted if there is blood present during oral sex.

HIV (and indeed hepatitis B, which is more infectious) can pose a small risk for both the active (person giving the oral stimulation) and receptive (person receiving oral stimulation) partner.
Transmission from an HIV positive receptive partner to an HIV negative active partner may occur when the active partner gets sexual fluid (semen or vaginal fluid) or blood (from menstruation or a wound somewhere in the genital or anal region) into a cut, sore, ulcer or area of inflammation somewhere in their mouth or throat. The linings of the mouth and throat are very resistant to viral infections such as HIV, so infection is unlikely if they are healthy.
Transmission from an HIV positive active partner to an HIV negative receptive partner is generally believed to be less common. This is because HIV it is normally only present in saliva in very low levels that are not sufficient to cause infection. The only risk in this scenario would be from bleeding wounds or gums in the HIV positive person’s mouth or on their lips, which may transfer blood onto the mucous membranes of the other person’s genitals or anus, or into any cuts or sores they may have. Hepatitis C can also be transmitted this way.

The risk of HIV transmission from an infected partner through oral sex is much smaller than the risk of HIV transmission from anal or vaginal sex. Because of this, measuring the exact risk of HIV transmission as a result of oral sex is very difficult. In addition, since most sexually active individuals practice oral sex in addition to other forms of sex (such as vaginal and/or anal sex) when transmission occurs, it is difficult to determine whether or not it occurred as a result of oral sex or other more risky sexual activities. Finally, several co-factors can increase the risk of HIV transmission through oral sex, including oral ulcers and wounds, bleeding gums, genital sores, genital or oral piercings, and the presence of other STDs.
When scientists describe the risk of transmitting an infectious disease, like HIV, the term "theoretical risk" is often used. Very simply, "theoretical risk" means that passing an infection from one person to another is possible, even though there may not yet be any actual documented cases. "Theoretical risk" is not the same as likelihood. In other words, stating that HIV infection is "theoretically possible" does not necessarily mean it is likely to happen - only that it might. Documented risk, on the other hand, is used to describe transmission that has actually occurred, been investigated, and documented in the scientific literature.
Various scientific studies have been performed around the world to try and document and study instances of HIV transmission through oral sex. A program in San Francisco studied 198 people, nearly all gay or bisexual men. The subjects stated that they had only had oral sex for a year, from six months preceding the six-month study to its end. 20 per cent of the study participants (39 people) reported performing oral sex on partners they knew to be HIV positive. 35 of those did not use a condom and 16 reported swallowing cum. No one became HIV positive during the study, although the small number of participants performing oral sex on HIV positive partners meant the researchers could only say that there was a less than 2.8 per cent chance of infection through oral sex over a year. In 2000, a different San Francisco study of gay men who had recently acquired HIV infection found that 7.8 per cent of these infections were attributed to oral sex. However, the results of the study have since been called into question due to the reliability of the participants' data.
In June 2002, a study conducted amongst 135 HIV negative Spanish heterosexuals, who were in a sexual relationship with a person who was HIV positive, reported that over 19,000 instances of unprotected oral sex had not led to any cases of HIV transmission. The study also looked at contributing factors that could affect the potential transmission of HIV through oral sex. They monitored viral load and asked questions such as whether ejaculation in the mouth occurred and how good oral health was. Amongst HIV positive men, 34 per cent had ejaculated into the mouths of their partners. Viral load levels were available for 60 people in the study, 10 per cent of whom had levels over 10,000 copies. Nearly 16 per cent of the HIV positive people had CD4 counts below 200. The study, conducted over a ten year period between 1990 and 2000, adds to the growing number of studies which suggest varying levels of risk of HIV transmission from oral sex when compared to anal or vaginal intercourse.
At the 4th International Oral AIDS Conference held in South Africa, the risk of transmission through oral sex was estimated to be approximately 0.04 per cent per contact. This percentage figure is a lot lower than the two American figures, because this figure is a risk per contact percentage, whereas the other figures are percentage risks over much longer time periods. Oral sex is still regarded as a low-risk sexual activity in terms of HIV transmission, but only when more work is done will we be clearer as to the risks of oral sex.

While it is very difficult to ever know how HIV transmission occurred, according to a fact sheet on oral sex produced by the CDC in 2000, there have been a few of documented cases of transmission during oral sex. These have occurred in both receptive and active partners during fellatio, cunnilingus and anilingus.

The risk of becoming infected with HIV from oral sex can be reduced still further by using condoms. Flavored condoms are available for those who don’t like the taste of latex or spermicide. For cunnilingus or anilingus, plastic food wrap, a condom cut open, or a dental dam (a thin square of latex) can serve as a physical barrier to prevent transmission of HIV and many other STDs.