present little risk. Having anal intercourse with an infected partner is the most likely way of catching AIDS, and 80 per cent of British cases so far have been male homosexuals.

The second most common way of contracting the disease is through infected blood. Almost a quarter of Britain's haemophiliac population now carry HIV because they have been injected with the clotting agent collected from infected blood. (Haemophiliacs are born without the blood-clotting factor, and can suffer severe bruising from a minor injury, and bleed to death from a cut unless they receive the clotting factor from donated blood.) HIV in the blood may also be transmitted on infected needles, and drug addicts are the third most highly at risk group of the population.

To become HIV positive you do not have to be homosexual or promiscuous, a drug addict or a haernophiliac. Heterosexuals are also at risk. Even a heterosexual in a steady relationship stands the risk of contracting the disease if their partner has been infected in a previous relationship. Therefore when embarking on any new relationship, it is safest to wear a condom.

Gonorrhoea is caused by the bacterium gonococcus, which cannot survive outside the body and is transmitted only by sexual intercourse, and never (as is sometimes imagined) on toilet seats or towels. In men the urethra, along which urine passes from the bladder, is infected, and there is sometimes pain on urinating and a thick discharge from the penis within a week after infection. In homosexual men the rectum may be infected, with the possibility of irritation and discharge from the anus. In women, gonorrhoea infects the cervix, urethra and rectum, and, as with men, there may be discharge and pain on urinating. If infection spreads to the uterus there is a 10 per cent chance that the fallopian tubes may be blocked, causing sterility. Often, however, there are no symptoms in either men or women.

The treatment for gonorrhoea is usually a single dose of antibiotics such as penicillin, with a check-up afterwards to make sure the infection has cleared. If gonorrhoea is not diagnosed and treated, serious complications can develop. Men may suffer epicliclymitis - pain and swelling  in the testicles; women may suffer peritonitis - inflammation of the membranes of the abdomen. Both sexes could develop gonococcal septicaernia, an infection of the bloodstream with skin rashes and arthritis. In serious cases, sterility can result in both sexes. Pregnant women with gonorrhoea may pass it on to their babies, who can be born with gonococcal opthalmia, an acute inflammation of the eyes. Complications are, however, relatively rare nowadays.

Non-specific urethritis or NSU can be identified by lumps, soreness or itching around the genitals, anus or mouth. There may also be a discharge from the vagina or penis. The treatment is usually a two-week course of antibiotics for anyone who has had contact with an infected person. During treatment, patients are asked to give up alcohol, as this can bring about a recurrence of the symptoms. Complications can occur, and these are similar to those for gonorrhoea, but fortunately early diagnosis and treatment can prevent these. It is possible for a man to be periodically reinfected with NSU without changing his sex partner, and no explanation has so far been put forward for this. However, both partners will need treatment each time NSU manifests itself.

Syphilis is quite rare in Britain today. It affects women less than men, and its main victims are male homosexuals. Symptoms appear between 10 days and 12 weeks after infection. In the primary stage of the disease a small hard sore or chancre appears on the penis, vagina or rectum. It is painless and usually disappears very quickly. A few weeks later, in the secondary stage, the patient is feverish, with swollen glands and itching skin. The disease is curable with antibiotics, but if for some reason it should not be treated, serious complications will develop many years later. Until the advent of antibiotics, tertiary syphilis used to be quite common, with patients eventually suffering from dementia and dying a slow, agonizing death.

Chlamycliais one of the most common STDs and is caused by a bacterial parasite called chlamydia trachomatis. The disease is diagnosed by a swab test, and treatment is with antibiotics. Symptoms in men include a whitish yellow discharge from the penis, frequent 'burning' urination, and redness at the tip of the penis. Women may notice a discharge, a frequent need to urinate, and mild discomfort which they may mistake for vaginitis or menstrual cramps. However, many women experience no symptoms until they develop complications such as pelvic inflammatory disease, a serious condition which can result in infertility. Babies born to infected mothers may suffer from eye infection, which is sometimes serious, or pneumonia.

Genital herpes is a viral infection transmitted through sexual intercourse. It is very similar to the other sort of herpes, which causes cold sores, and can also be caught by having oral sex with someone who has active cold sores. The symptoms are itching, pain in the groin, discomfort on urinating and fever, followed by the appearance of painful red blisters on the vulva or penis, which burst to form ulcers. After about 10 days the symptoms disappear and the patient appears to be cured. But the infection is only lying dormant and may recur at any time, particularly when the patient is under stress. There is as yet no treatment for this disease. While the disease is dormant, it is safe to have sex without infecting one's partner, but it is impossible to predict the next attack, so the risk of infection remains. If the infection is active at the end of a pregnancy, a Caesarean section may be performed to prevent the baby becoming infected in the birth canal.

Genital warts are unpleasant but painless and can be treated quite easily. They are small lumps that appear on the penis, vulva, or anus and are mildly contagious. The treatment involves either painting the warts with a preparation called poclophyllin, or freezing them off with liquid nitrogen. An association has been identified between genital warts and cervical cancer, so it is important to get rid of them as soon as possible, and to have regular cervical smears.

Thrush is a fungal infection that develops in certain conditions in the vagina. It is sometimes linked to taking the Pill, and if it recurs frequently, a different method of contraception may be advisable. A man may carry thrush, though he usually manifests no symptoms. Thrush causes vaginal soreness and itching, and a thick white discharge. The doctor will probably prescribe anti-fungal cream, to be used by both partners, and vaginal pessaries, though oral treatments are available too. Some women find that natural yoghurt in the vagina is effective. Avoid hot baths, and wearing tights, tight jeans and nylon knickers.