But keeping that system in good shape has never been more important. Every winter brings waves of new cold and flu bugs, and with experts warning that a flu pandemic may appear at any time, it's vital to be aware of the best ways to give you and your family's immune system a boost

This is when you're far more prone to infection, so take active steps to protect yourself. ‘Just increasing your levels of hygiene around the house can help break the chain of disease transmission,' explains Professor John Oxford, Consultant in Infectious Diseases at St Mary's Hospital, London. ‘We catch these viruses in two ways. We either breathe them in after someone coughs or sneezes, or we touch a surface where they have settled, then transfer them to our eyes, nose or mouth. Viruses can survive for several hours, so good hygiene is important.'

Wash your hands frequently and thoroughly - not just the palms but also the backs of your hands and in between your fingers - and make sure your children do the same. Studies have shown hand washing can reduce the spread of colds within families.

Wash surfaces, door handles and telephones frequently with hot, soapy water or an antibacterial cleaner.

Avoid touching your face. The viruses enter the body via the eyes, nose and mouth.

Get anyone who's ill to use paper tissues when sneezing or coughing, and bag and bin them after use. Each sneeze or cough can project up to 100,000 infected droplets as far as 10m.

Be vigilant all the time. People are infectious for at least a day before symptoms appear, and then for three to five days after.

Long-term stress compromises your immune system and the longer it lasts, the more profound the impact - it's thought that ongoing stress leads to the release of hormones that disrupt the production of some of the body's key disease-fighting cells. But it's not just work pressures that can affect your immunity - social stresses are damaging, too. A study of first-year college students in Pittsburgh found that those who felt lonely or isolated had a lower immune response to a flu jab than the more gregarious students. Similarly, elderly people caring for relatives with Alzheimer's were found to make fewer antibodies after a flu jab than non-carers.

It might sound easier said than done, but studies have shown that any kind of stress reduction or relaxation time can help to maintain immunity. Recent research found that people who did eight weeks' meditation training had a stronger immune response to a flu vaccine, while a study by doctors at the University of California found that 15 weeks of t'ai chi exercise boosted elderly people's levels of immunity against shingles by 50%. Another study at Ohio State University revealed that using self-hypnosis to relax boosted immune function in a group of medical students facing exams. The more frequently they used the technique, the better their immune response, while students not practising self-hypnosis showed a drop in immune function.

Experts think it's protective because it slows the release of stress hormones, which are known to lower immunity, and boosts circulation, which allows antibodies and white blood cells to move more quickly through the body, detecting and destroying harmful bugs. Research suggests people who exercise regularly suffer fewer colds and minor infections, and generally have a measurably stronger immune response than couch potatoes.

Although we all accept that winter is the cold and flu season, the idea that being cold could mean you'll catch a cold has always been dismissed as an old wives' tale. Now, however, researchers at Cardiff University's Common Cold Centre have found out that getting cold can actually make you more likely to become ill. A group of volunteers had their feet chilled in icy water for 20 minutes and another group had theirs in an empty bowl. Far more of those with chilled feet developed cold symptoms over the next four to five days than the others. The reason, says Centre director Professor Ron Eccles, is that being chilled reduces your immune defences, allowing dormant infections to take hold. ‘When colds are circulating,' he says, ‘many people are mildly infected but show no symptoms. If they become chilled, this causes a pronounced constriction of the blood vessels in the nose. The reduced defences in the nose allow the virus to get stronger and cold symptoms develop.'

Wrap up warm, and cover your nose and mouth when you go out in cold weather. ‘We wrap ourselves up in winter coats to keep warm but the nose is directly exposed to the cold air,' says Professor Eccles. ‘Cooling of the nose slows down both the clearance of viruses from the nose and the response of white cells that fight infection.'

The combination of too little sleep and drinking more alcohol than normal can wreak havoc with your immune defences. Heavy drinking interferes with the breakdown and absorption of many vital nutrients, which in turn weakens immunity, leaving you more prone to infections. It's not just long-term heavy drinking that's a problem - even isolated episodes of binge drinking can temporarily depress immunity. Adding smoking to the equation is even more damaging. Smoking has been shown to suppress the activity of the body's immune cells and deplete vitamin C levels. What's more, research shows that heavy smokers develop more severe colds, and get them more frequently, than non-smokers.

Slow down and look after yourself. Check that your drinking levels are within safe limits (a maximum of 14 units a week and no more than two to three drinks daily), allow yourself at least seven hours' sleep every night, eat properly and don't smoke. Just 30 days after quitting, your immunity will improve.