The right diet and plenty of exercise can help keep the snivels out of the classroom, says Helena Pozniak

It's half-way through the first term back at school. Gone are the tanned, happy-go-lucky children of last summer, back are the pasty, nose-picking, nit-scratching, reluctant pupils, coughing and sniffing their way to the playground as the winter months draw closer. If your child doesn't bring his own germs back to school at the end of half-term week, he'll almost certainly find someone else's to bring home.

"Children do have an unhealthy time when they go back to school. Suddenly they are mixing again with a 200-or-so strong peer group, with little resistance to bugs and viruses they have never met before. Of course they are prone," says Dr Graham Archard, vice chair of the Royal College of General Practitioners.

Keeping your children healthy sounds deceptively simple. There are two mainstays for a good immune system - not getting over-tired and having a well-balanced diet. Yet speak to any parent by the end of term and discover more often than not their child is exhausted, irritable and won't even look at vegetables.

If you want your child to sleep better, exercise is key, says Dr Archard. "It does several things - your child will sleep better at night and protect his or her immune system." Physically active children have fewer chronic health difficulties than their sloth-like peers, are less likely to become obese and have more self-confidence. Yet parents can't always rely on schools for physical activities - recent surveys show a sorry lack of sports facilities. As an alternative to conventional sports, throw a frisbee, play hide and seek, kick a ball around or race in a park. Now that the weather has turned, try mini-discos, games of twister, hula-hoops or balloon volleyball.

Succumbing to the odd cold or bout of diarrhoea doesn't mean you have a sickly child. "It's perfectly healthy for a child to get a cold," says nutritional therapist Jayne Nelson. "The high temperature is killing the virus - your body is responding as it should. But if your child is continually ill, your immune system may be out of balance."

It's not unusual for half a class or more of young children to be struck down by the rotavirus - which is one of the main causes of gastroenteritis in young children and prevalent during winter. Almost every child in the country will have suffered a bout before his or her fifth birthday, while parents rarely catch it as immunity lasts into adulthood.

Most doctors agree that encouraging children to wash hands at various stages of the day with simple soap and water cuts all kinds of risks of infection from sickness and vomiting, to colds, flu as well as more serious infections such as meningitis. But keep hygiene measure in perspective and avoid anti-bacterial soaps, health experts say - children need to encounter a bug or two to beef up their immune systems.

So much has been written about the sorry state of children's diets that the five-a-day guideline begins to sound like a mantra and no parent can claim not to know what children need to eat. While the Government has pledged £220m to improve school lunches, some schools have attacked junk food with a vengeance - the headmaster of a private school in Scotland has ordered teachers to confiscate chocolate bars, fizzy drinks and unsuitable snacks. Healthy eating policies intend to make the drop in blood sugar levels - which has been found to occur at around 10.45am in primary school children, bringing the associated headaches, dizziness, fatigue or nausea - a thing of the past.

"Plenty of colour and variety in your diet and you will get all the nutrients," says Nelson. "Most nutrients need replacing daily." Breakfast is often our worst balanced meal of the day, but the most important. "Toast, marmalade and undiluted orange juice is just a pure sugar fix," warns Nelson. "Even sausage and beans is a better alternative." Parenting websites are thick with healthy breakfast suggestions - oatcakes spread with nut butter and jam and an apple, for example - by adding fruit to every meal, you achieve a higher level of nutrition.

Certain foods can boost immunity, nutritionists believe - raw garlic, live yoghurt, walnuts, broccoli and most fruits for example. In her book Immunity Foods For Healthy Kids, nutritionist Lucy Burney details meals for fighting specific illnesses as well as foods to boost the immune system of babies to teenagers.

Essential fatty acids - omega 3 and omega 6 - are imperative for good health, normal growth and are beneficial for the brain - some studies have shown they improve concentration and behaviour of children who under-perform in class. They must be eaten regularly, as the body has limited storage. They're found in oily fish such as mackerel, herring, sardines, salmon and tuna (fresh or frozen, not tinned), nuts and seeds. "There are plenty of well-researched good supplements," says Nelson.

One of the most powerful ways of encouraging children to eat healthily is peer pressure, says psychologist Professor Fergus Lowe. So invite good eaters back for tea and lavish praise on them, experts suggest and set a good example yourself.

Amid the furore over the last few years over vaccinations - the MMR in particular - several groups of school age children still find themselves unprotected from measles, mumps and rubella. "Mumps is currently a problem," says Helen Stretch, school nurse in Ceredigion. "Pupils are more vulnerable in secondary and further education." In the majority of cases, mild mumps causes swollen glands around the face, but it can cause swelling in the testes and in severe cases, make boys sterile. It's also one of the main causes of viral meningitis - symptoms can include a severe headache, sudden high temperature, sensitivity to bright lights, vomiting, painful joints, fitting or drowsiness. Acute infections such as these are unlikely, and most school nurses and teachers are clued up to spot symptoms.

More common and less preventable are the host of irritating infections such as the fungal athlete's foot, impetigo - a bacterial skin infection causing pus-discharging sores, or the ubiquitous head lice. "Vigilant families keep their children lice free over the summer," says Mrs Stretch, " but back among the school children, they are continually infected, and there's not much you can do about it." Nits are the white eggs left on the hair after the lice hatch - they can crawl from head to head but can't jump, swim and don't live long on pillows. Either treat with chemical products - or cover your child's hair with conditioner twice a week and use a nit-comb to tease them out.

Some doctors feel the focus on diet has hijacked attention from the most important role parents can play in their child's health - looking after their social health. "It's not just about worrying if your child is eating crisps," says Dr Archard. "Use meal times to find out how your child is getting on with his or her peers - talking to them is the only way you get to identify if anything's wrong. Children's social well-being is extremely important and often overlooked."