Threadworms, like headlice or bad breath, are seldom discussed in polite company. Yet between two and three million children in Britain play host to these nasty little parasites, which have extremely cunning ways to ensure their survival.

Although worms seldom cause more than an itchy bottom, most parents are horrified when they discover that their children are affected. In some cases scratching can result in soreness and painful inflammation. Occasionally threadworms can migrate into the vagina, leading to a discharge or cystitis.

Many people find the idea of worms inhabiting their children's insides a repulsive one.

'It is not a life-threatening condition, but it is irritating, in every sense of the word, and distressing for children and parents, who can also become infected,' says Professor Donald Lee of the department of pure and applied biology at Leeds University.

The worm's life-cycle starts in the large intestine, where the male fertilises the female. When the female is ready to produce her eggs - all 11,000 of them - she works her way down to the anus, where she lays them on a sticky secretion, usually during the night.

This secretion, and the worms themselves, cause itching, so that the poor victims inevitably scratch themselves. The result is that the worms' eggs get under their fingernails and into their mouths, either because they suck their fingers or eat meals without washing their hands first. The eggs then pass down to the intestine, where they hatch - and so the cycle continues.

The female's life-cycle is particularly unattractive, since she sometimes bursts because of the pressure of the eggs inside her. The male's life is happy but short: after fertilising the female he dies.

Professor Lee said that the worms may also creep across into the vulva region, which can be particularly upsetting for girls and their mothers.

Dr Ann McPherson, an Oxford GP and co-author of Mum, I Feel Funny, which deals with this problem, says: 'We see loads of threadworm. We treat it with a one-off dose of medication which kills both the worms and the eggs in the intestine. It is a good idea to treat the whole family because it does get passed round.

'I usually advise the family to take another dose, two weeks later, to avoid recurrence. The medication can be bought at the chemist without a prescription, if parents want to do so.

'Everyone should wash their hands after going to the toilet and before eating, and scrub under their fingernails.

'To get children not to scratch when they are itchy is impossible, but we often tell them to try and scratch through their pyjamas, and some doctors tell children to wear pants in bed,' she says.

'If you itch, think worms,' is Dr McPherson's general advice.

Diagnosis is relatively easy. Parents may actually be able to see the worms in their children's stools, if they can persuade them to do them in a pot. The worms look like short, white threads, about a centimetre long and half a millimetre wide.

The other method is to stick a piece of sticky tape over the anus, when the child goes to sleep at night. If they have worms, the tape will catch the eggs. The GP will then send the tape to a public health laboratory for analysis.

No one knows exactly how many people are infected, because it is not a notifiable disorder and not often talked about, but estimates vary from 20 to 30 per cent of the childhood population, which works out at between two and three million. The group most at risk are those aged five to nine.

Experts differ about what hygiene methods should be adopted when a child becomes infected. Professor Lee says that because the worms' eggs get everywhere - on bedclothes, the carpet, ordinary clothes, even in the dust in the air - strict hygiene measures should be adopted.

'It requires frequent laundering of the nightclothes, bedclothes and underclothes in a hot wash, good personal hygiene, frequent bathing and cleaning of the bedroom with a vacuum cleaner, because that sucks up the eggs, rather than scattering them, which is what a brush does,' Professor Lee says.

But other doctors are less ambitious. Dr Aidan Macfarlane, consultant community paediatrician for Oxfordshire district health authority, says: 'If you can get the whole family to scrub their hands and take the medicine, you are doing well. Like headlice, threadworms are always around.'