While catching a cold is not the end of the world, it is often inconvenient and unpleasant. It is also, to some extent, avoidable.

* Avoid sugar, especially if you feel you are coming down with something. Sugar lowers immune function almost immediately, and may make the difference between fending off a cold and succumbing to it. Sugar is present in foods you may not suspect, like ketchup and fruit juice.

* Get enough sleep - it will keep you strong and ensure that your body has the strength to fight off any potential invaders.

* Take vitamin C. Not all studies agree, but the weight of the evidence does seem to suggest a protective effect, reducing the incidence of the flu and colds by 28 and 50 per cent, respectively (Deutsche Gesundheitswesen, 1954; 9: 702-6; Int J Tuberc Lung Dis, 1999; 3: 756-61). Those under heavy physical stress will particularly benefit (Int J Sports Med, 1996; 17: 379-83). Aim to take 1000 mg daily.

* Other useful supplements include vitamin A (8-12,000 IU daily), which can reduce the in-cidence and severity of infection and illness; vitamin E (200-400 IU daily), which boosts resistance to infections; and coenzyme Q10 (20-200 mg daily), which can compensate for immune deficiencies due to ageing or disease. Probiotics, though technically not nutrients, do improve immune function (Int J Immunother, 1993; 1X: 23-8). Supplements should contain at least three billion viable bacteria per capsule.

* Exercise improves circulation of immune complexes. Regular moderate exercise can lead to fewer colds and flu (Exerc Immunol Rev, 1997; 3: 32-52; J Gerontol A Biol Sci Med Sci, 2002; 57: M557-62). But don’t overdo it since pushing yourself past your limits may have a paradoxical immune-weakening effect (Int J Sports Med, 1994; 15: S131-41).

* Take garlic regularly as it is antibacterial, antiviral and antifungal. Fresh is best, used either in cooking or added to other ingredients in a juicer. But supplements are also effective (and less smelly). In one recent trial, one capsule per day of a garlic supplement containing stabilised allicin for 12 weeks led to 63 per cent fewer colds and 70 per cent fewer sick days than those taking a placebo (Adv Ther, 2001; 18: 189-93).

* Think twice about the flu vaccine - it does not work (Cochrane Database Syst Rev, 2001; 4: CD001269) and may even weaken your immune system. As much as 30-40 per cent of elderly people don’t respond to it (Hum Immunol, 1994; 40: 202-9) and around one in 12 patients (8 per cent) say it triggers asthma (Lancet, 1998; 351: 326-31). Other adverse effects include optical neuritis and permanent blindness, vasculitis and joint problems, reversible paralysis and myelopathy (see also box below).

* Cover your mouth and wash your hands. Colds/flu are spread chiefly through the air (Eur J Epidemiol, 1987; 3: 327-5). When you cough or sneeze, infectious microbes are expelled at over 120 miles per hour, easily reaching most people in your vicinity. Washing your hands with regular (not antibacterial) soap will also decrease the likelihood of spreading a virus to your nose, mouth or other people.

* Adaptogenic herbs such as Siberian or Asian ginseng, Astragalus, Schisandra and Rhodiola are thought to help keep various systems - including immune function - healthy, and are often recommended to prevent colds/flu. In one trial, people given 100 mg of Asian ginseng plus a flu vaccine had fewer colds/ flu compared with those receiving only the vaccine (Drugs Exp Clin Res, 1996; 22: 65-72).