Vitamin A is found in cod liver oil, which is also rich in vitamin D, and meat, oily fish, cheese, and whole milk and eggs, which are also rich sources of retinol, the active form of vitamin A.

Taken as a supplement, an effective dose is between 8000-12,0000 IU/day.

- Vitamins C and E, which are antioxidants like vitamin A, are also well established as immune-system enhancers. Vitamin E is especially effective for reducing susceptibility to infections (Arch Immunol Ther Exp, 1987; 35: 207-10).

A dose of around 200-400 IU/ day of vitamin E, and 1000 mg/day of vitamin C, is recommended to boost the immune system.

- Zinc is another potent antiviral. In one study, a zinc nasal gel significantly reduced the duration of a cold (Ear Nose Throat J, 2000; 79: 778- 80). The more popular zinc lozenges appear to be less effective (J Am Med Assoc, 1998; 279: 1962-7).

- Echinacea is supposed to be one of the most effective immune boosters among the herbal remedies, although the hard science to back the claims of its advocates is inconclusive (Arch Farm Med, 1998; 7: 541-5). Like zinc, Echinacea seems to be at its most effective if taken during the first hours of infection (J Fam Pract, 1999; 48: 628-35).

- Hydrastis canadensis, or goldenseal, is another herbal standby but, again, the science behind it does not support its high reputation.

- Andrographis paniculata, or Kalmegh, an Ayurvedic remedy, is a more effective herbal that has been successfully tested against placebo in two studies (Phytomedicine, 2000; 7: 341-50).

- Elderberry is an effective antidote to the flu. In one study, 60 flu victims took either 15 mL of elderberry extract or a placebo, four times a day for five days. The elderberry relieved symptoms four days earlier than did the placebo (J Int Med Res, 2004; 32: 132-40).

- Phytolacca americana, or pokeweed, is another herb that is known to stimulate the immune system - and it’s especially effective against flu viruses (Antimicrob Agents Chemother, 1980; 17: 1032-3), although it can be toxic if taken over the long term.

* Exercise

- Regular, moderate exercise is good for improving immune function (Exerc Immunol Rev, 1997; 3: 32-52). But bursts of heavy exercise may have the opposite effect, and can suppress the immune system for several hours, thereby increasing the chances of an upper respiratory tract infection (Int J Sports Med, 1994; 15: S131-41). It’s worthwhile noting that the soldiers who were most badly affected by the swine flu outbreak of 1976 were just beginning their basic combat training, a time of exceptional exertion (J Infect Dis, 1977; 136: S363-8).

* Stress

- High stress levels probably have the single most important influence over the state of the immune system (J Fla Med Assoc, 1993; 80: 409-11). This was shown in a study where the participants - half of whom were proven to have high stress levels, the other half with low stress levels - were all exposed to rhinoviruses, the viruses responsible for the common cold. Only 27 per cent of those in the low-stress group developed symptoms, compared with 47 per cent of the high-stress group (N Engl J Med, 1991; 325: 606-12).

* Prescription drugs

- You are much more likely to develop the flu if you are taking a prescription drug. Antibiotics can enhance flu susceptibility (J Am Med Assoc, 1997; 278: 901-4), including flucloxacillin (Med J Aust, 1989; 151: 701-5) and Septrin (trimethoprim, sulphamethoxazole) (Br J Dermatol, 1987; 116: 241-2).