There have been several studies reporting transmission of HIV between gay men who exclusively practice fellatio, rather than penetrative intercourse. There has also been at least one report of female to female transmission. In heterosexual couples, there is a significantly greater risk of transmission of HIV from male to female, than from female to male, but this includes all sexual practices, not just oral sex. As few couples exclusively practice oral sex, it is difficult to get reliable evidence and quantify the risk from oral sex alone, but from our understanding of how the virus is transmitted, we believe that the receptive partner (i.e. the one taking the penis into the mouth) is most at risk. Taking semen into the mouth, particularly with poor oral hygiene, further increases the risk for the receptive partner, although there are reports of HIV transmission as a result of fellatio without ejaculation.

Bacterial infections of the digestive system
There is evidence that the organisms Salmonella, Shigella and Campylobacter can all be transmitted by oro-anal contact. These can all cause severe abdominal pain and, sometimes, diarrhoea. Infections with Salmonella and Shigella can occasionally prove fatal.

Chlamydia
Chlamydia trachomatis infection may cause urethritis in men (pain and discharge from the urethra - the outlet from the bladder) or, in women, pelvic infection and fertility problems. In women Chlamydia quite frequently causes no symptoms. The organism has been isolated from the throats of both men and women. There is an association between fellatio and chlamydia infection, suggesting that at least male to female transmission is possible.

Gonorrhoea
Gonorrhoea is a sexually transmitted disease caused by the bacterium, Neisseria gonorrhoea. It usually causes a urethral (penile) discharge and discomfort on passing urine in men, and vaginal discharge and pelvic pain in women. It can be present without producing any symptoms at all, particularly in women, where up to 70 per cent may have no symptoms of infection. It is normally transmitted by vaginal or anal intercourse, but can also be transmitted through oral sex. It can cause gonococcal pharyngitis (throat infection). This is frequently without symptoms and only about 15 per cent of those affected will have a sore throat. Gonorrhoea is highly infectious.

Hepatitis A
Hepatitis A is a common viral infection that can cause jaundice and abdominal pain. It is not usually serious, although sufferers can feel very ill. The virus is found in faeces in high concentrations and will almost inevitably be present on the apparently clean anal skin of infected individuals. It can be transmitted by oro-anal contact. Several epidemic outbreaks have been reported among gay men, but heterosexual couples practising oro-anal contact are just as likely to be at risk.

Hepatitis B
Hepatitis B is a viral infection that is more common among people who have had a previous sexually transmitted disease, and among drug users. It can cause a very serious, potentially fatal, liver disease and chronic liver damage. It is most commonly transmitted by inoculation of infected blood, by sharing needles for injection, needlestick injuries and the medical use of infected blood products. Virus particles are found in semen, stool and saliva, as well as blood. There is clear evidence that it can be transmitted through vaginal and anal intercourse, but it is unproven whether it can be transmitted through oral sex. Certainly, there is a theoretical risk of transmission.

Hepatitis C
Hepatitis C is a viral infection transmitted in a similar way to hepatitis B, and so usually affects drug users. Some people with hepatitis C infection were infected with blood products, such as transfusions, prior to adequate screening procedures in the 1980s. There is limited evidence that hepatitis C may be transmissible through receptive oro-genital contact.

Herpes simplex virus (HSV)
HSV infection is the commonest cause of genital ulceration. In the past, it was almost exclusively caused by Herpesvirus hominis type 2, but in recent years it is increasingly caused by the type 1 virus, which was more usually associated with labial (lip) herpes or cold sores. Genital herpes is characterised by recurrent bouts of vesicles (small blisters), either on the penis or vulva, or other parts of the female genital tract. These rapidly break down to form small, painful ulcers. The first episode is usually associated with an acute feverish illness, which may be quite severe. It frequently recurs, although recurrent bouts are usually associated with a milder illness. HSV can also cause pharyngitis, an inflammation of the throat with similar ulcer formation. HSV is highly infectious and usually sexually transmitted. It is certainly transmitted by penetrative intercourse but there are several reports of transmission through oral sex. Virus particles are shed profusely from ulcers, either oral (including cold sores) or genital, and infection is far more likely when these are present. Transmission is unlikely, but not impossible, in the absence of ulcers.

Human papillomavirus (HPV)
HPV infection leads to the development of genital warts in both men and women, and is almost certainly sexually transmitted. Warts are usually fairly obvious on men (present on the penis), but may be more difficult to diagnose in women, as they may be restricted to the cervix (neck of the womb) and vagina. They can also develop around the anus, particularly if anal intercourse or oro-anal sex is practised, and, rarely, in the mouth and throat. HPV infection is important to identify and treat, as it can lead to the development of cancer of the cervix, mouth, penis and anus.

HPV virus is shed from the surface of warts and any form of direct physical contact may result in transmission. Although there is no definite evidence that HPV is transmitted through oral sex, it is highly likely that this can occur.

Molluscum contagiosum
Molluscum contagiosum is a common, infectious viral disease affecting the skin. It appears as multiple, small, dome-shaped blebs, often with a central plug. A curd-like discharge can be squeezed from them. The vast majority of molluscum lesions are transmitted in ordinary contact  they are common in children for example. There is limited evidence that it may be transmissible through receptive oro-genital contact.

Syphilis
There is a risk of transmission of syphilis by oro-genital sex, although it is difficult to quantify. Certainly, men and women may develop syphilitic mouth ulcers, or chancres, which shed the bacterium that causes syphilis, Treponema pallidum.

Thrush
Thrush is caused by a yeast, Candida albicans. Many people, perhaps up to 50 per cent of the population, carry Candida in their mouth or on the skin without it causing any symptoms. However, it can also cause infection and inflammation of the mouth, vulva, vagina, head of the penis and foreskin, usually with a cottage cheese-like white discharge present. Injury to the skin or mucous membranes, contraceptive pill use and diabetes are all predisposing factors. There is limited evidence that Candida can be transmitted by oral sex.

Worms
Threadworms, and probably other worms, can be transmitted by oro-anal contact or by fellatio after anal intercourse.

Risk reduction
Please note we are discussing 'risk reduction' and not 'risk prevention'. Avoiding oral sex is the only way of preventing infection by the oral route, but as it is an important and enjoyable part of most couples' sexual repertoire, this advice is impractical. Overall, oral sex seems to carry a lower risk of transmitting infections than penetrative vaginal or anal intercourse.

Limiting the number of your sexual partners and not being pressured into 'casual' sex with an unfamiliar partner is an important way of reducing infection risk. Consider whether your partner may have, or have had, other sexual partners, and what their infection status might be, before putting yourself at any risk.

It is perfectly possible to enjoy oral sex using some form of protective barrier. For fellatio, there are a wide variety of flavoured and scented condoms available, ranging from strawberry to curry, and from whisky to lager and lime! For cunnilingus and oro-anal sex, a dental dam - a thin plastic film - can be used. Remember that condoms can burst and dental dams can slip, so don't think that their use offers absolute protection. These products can be obtained from most pharmacies and many supermarkets. They can also be obtained by mail order from the Family Planning Association. Write to them to request their SexWares catalogue, at The Family Planning Association, PO Box 883, Mail Order Department, Oxford OX4 5NT.

Even if you use a barrier of some sort, it is important to maintain good oral hygiene and not to have oral sex if you have any cuts, sores or ulcers in your mouth, or on your gums or genitals. If you do get semen or vaginal secretions in your mouth, either spit them out or swallow them quickly. Don't keep them in your mouth, as it is the mouth that is probably the most important route for transmission of infections. The longer they are in your mouth, the greater the chance of infection.

If you are concerned that you have, or might be at risk of having a sexually transmitted disease, it is important to seek medical advice as soon as possible. Effective treatment is available for most diseases, including HIV. Early treatment is not only important for your own health, but in order to protect your partner from infection.

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