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As Autumn sets in, so begins the season of colds and flu. As many as 15 per cent of the population will have had flu by February, and the likelihood of you becoming a statistic of the season depends partly on how well your immune system is working. For example, for each person who has a cold, there are two or three others who are infected but don't succumb.

Professor John Oxford, a specialist in virology from Queen Mary School of Medicine, London, says: 'We are surrounded by numerous bacteria and viruses every minute of the day.

A poor diet, over-indulging in alcohol and smoking can all affect the immune system, but there are steps you can take to improve it - some of them rather surprising...

Eating adequate amounts of protein, as well as plenty of fruit and vegetables, is a simple way to improve the immune system, says Catherine Collins of the British Dietetic Association, chief dietician at St George's Hospital in Tooting, South London.

The average adult needs 40g to 60g (11/ 2oz to 2oz) of protein daily. Just 35g (1oz) of meat provides around 7g of protein - as does an egg, an ounce of cheese or a glass of milk. Meat is also rich in selenium and zinc, two of the minerals needed for a healthy immune system.

Offal is rich in selenium. Other sources include Brazil nuts, cashew nuts and shellfish. Zinc is found in most red meat, pumpkin and sunflower seeds, milk, cheese and wholegrain cereals.

Granny was right - wrapping up warm when there is a chill will help protect you against colds and flu. But scientists now believe it is especially important to keep your nose warm.

'The more the temperature drops, the more the incidence of colds and flu rises, and this is not because we tend to crowd together as some people previously thought - it is because we get colder,' says Professor Ron Eccles, director of the Common Cold Centre at Cardiff University.

Shivering has been found to greatly increase the risk of a cold, so staying warm is vital. Wear a scarf over the nose and thick socks in cold weather.

While many people think they feel better after a good night's sleep, it's sleeping less that boosts the immune system. Studies have found that getting less sleep than you do normally for a couple of days boosts the immune system briefly. But if you are stressed, a lack of sleep will make you vulnerable to colds and coughs.

Swallowing lots of vitamin pills is unlikely to reduce your risk of getting an infection and may even harm your health. 'Getting nutrients from food is far more beneficial than taking the same nutrients in tablet form,' says Anna Denny, nutritional scientist at the British Nutrition Foundation.

'We used to believe it was just the vitamin content of fruit and vegetables that made them so beneficial to health, but now we know that they contain a range of phytochemicals which are just as vital.

Boosting your intake of omega 3 could help reduce your risk of infection. The problem is we tend to eat too much omega 6 instead, says Dr Ann Walker, a senior lecturer in nutrition at Reading University. Both omega 3 and omega 6 are vital to good health.

However, the Western diet has vastly more omega 6 oil - found in processed sunflower oil, certain margarines, corn and sesame oil - than omega 3, which is found mainly in oily fish such as mackerel, seeds and rapeseed oil. The average person in the UK eats omega 3 and omega 6 oils at a ratio of 1: 10 when it should be 1: 3 or less.

We should eat two portions of oily fish a week or otherwise take a daily supplement of omega 3 oil. Using mono-unsaturated olive oil rather than omega 6-rich sunflower oil will also help reduce your omega 6 intake.

T'ai Chi is an ancient martial art from the Far East, which combines gentle exercise and relaxation to increase concentration as well as develop strength and flexibility. It has been found to improve immune function by 50 per cent.

Short bursts of stress actually strengthen the immune system. When we start to become stressed we release the hormones cortisol and adrenaline. This 'fight or flight' response warns the body of the possible imminent threat of infection.

The immune system responds by increasing the production of immune cells. So tomorrow, give yourself a shorter deadline for an office task; offer to make a presentation or speech; or maybe invite 12 friends for a four-course meal this weekend. The key is doing something stressful that you know will end soon.

Long-term stress, however, has the opposite effect. Offices where the stress levels are high have worse rates of infections than those of more relaxed companies. Hobbies are also beneficial. Bridge players, for example, have been found to have a stronger immune response after a game.

Women with a history of gaining and losing weight have fewer white blood cells, the so-called killer cells which are one of the main defences of the immune system.

People who have regular sex have more bacteria fighting antibodies than those who are celibate, according to U.S. research. It is thought that this is because sexually active people are exposed to infections which help keep their immune system primed.

A study found that people who took a daily probiotic had 2.5 fewer days off sick with colds and tummy upsets than people who just took a multivitamin and mineral.

German research found that a daily probiotic reduced the average duration of a cold by two days. Probiotics contain bacteria such as lactobacillus that help maintain a healthy bacteria balance in the gut.

'There is evidence that maintaining the immune system of the gut by taking probiotics helps improve general immune function,' says colds expert Professor Ron Eccles.

'We know from studies that people with a couch-potato lifestyle get more infections than people who have a physically active life,' says Professor Mike Gleeson, professor of sport science at Loughborough University.

Researchers have found that people who have two hours daily of moderate activity (including physical chores) have 30 per cent fewer respiratory infections.

'Exercise also increases the concentration of antibodies in the saliva which helps protect you against respiratory infections,' says Prof Gleeson.

While moderate exercise gives a temporary boost to the amount of white blood cells in the body, people who spend one and a half hours or more working out will have the opposite effect. So exercise, but not too much.

Drinking tea has been found to help strengthen the immune system. Scientists have found that a chemical in tea called alkylamine antigen is also present in some bacteria cells. Exposure to the chemical helps the body build up defences against the bacteria.

American research has shown that the immune cells of volunteers who drank five small cups of black tea daily were better able to fight off disease than the immune cells of volunteers who drank five cups of coffee daily.

While the vitamin is found in certain foods such as oily fish, milk and fortified margarines, around 90 per cent of our supply is produced by the action of the sun's rays on our skin - indeed, our vitamin D levels are far higher in the summer than winter, and it is estimated that as many as 60 per cent of the UK population is deficient in it during the darker season between October and March.

While washing your hands will not actually strengthen your immune system, it will drastically reduce your chance of catching colds and flu. 'About half of all colds and flu are spread by hand-to-hand contact,' says Prof John Oxford.

You can buy special hand washes but they offer only limited protection. 'The current ones on the market will kill off viruses and bacteria on your hands but that protection does not last and as soon as you touch something which is infected, you will be liable to infection once more,' warns Prof Eccles.

While recent research suggested echinacea could not prevent colds, other studies have found that taking North American ginseng can dramatically reduce your risk.

In a study of 130 people in Canada who regularly suffer two or more colds a year, the people who took two 200mg capsules of ginseng root a day had 31 per cent fewer colds than those who took a placebo.

'It appears that the ginseng atoms look like bacteria atoms so they keep the immune system in action. Whether it is right or necessary to keep the immune system primed in this way is a point of debate,' says Prof Eccles.

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