"Almost all vitamins and minerals play some role in ensuring an optimal immune response...but high doses do not help and may do harm," says Catherine Field, dietician and professor of nutrition at the University of Alberta.

Here are the vitamins and minerals to fight viruses and in which food they are found most based on evidence provided by Field.

Vitamin C: Optimal vitamin C status has been identified as important for the immune cells involved in defence against viruses.

The main function of vitamin C is to help heal cuts and wounds; keep gums, teeth, and bones healthy; keep blood vessel walls strong and help absorb iron from the foods we eat.

Despite being studied for over 40 years, there is insufficient evidence to advise taking more vitamin C to prevent colds or the flu. The Recommended Daily Allowance (RDA) is 75 mg for women and 90 mg for men (an additional 35 mg should be added for smokers) which is far too low for most people.

This is easily obtained by having one to two servings of vitamin C rich citrus fruits (such as oranges), or vegetables like sweet peppers and broccoli. A higher dose of vitamin C is not without side effects such as causing digestive problems.

Zinc: Zinc is important for the cells involved in defence against viruses. Zinc is also involved in many bodily functions. It supports normal growth and development during pregnancy, childhood and adolescence.

It is also required for a proper sense of smell and taste so that low zinc status can influence your appetite and enjoyment of food.

The current RDA for zinc is eight mg for women and 11 mg for men. The best sources of zinc are seafood, meat, seeds, cooked dried beans, peas and lentils.

A serving of lentils (3/4 cup) provides almost two mg of zinc. Plant sources are less reliable as the level of zinc in plants depends on the content in the soil.

As a result, vegetarians who mainly depend on plant sources of nutrients are advised to consult a dietician to ensure their needs are being met.

Selenium: Although selenium is important for a healthy immune system, there is little evidence that consuming selenium supplements will reduce the risk of viral infections.

Recommended amounts are small, only 55 micrograms daily for adults, readily obtained from nuts, seafood, organ meat, pork and whole grains. Half a cup of cooked brown rice provides eight to 10 micrograms of selenium and a serving of mixed nuts (or 1/4th cup) has about 150 micrograms of selenium.

Viral infections, such as the flu, are often associated with a fever. However, there is no evidence that "starving a fever" by reducing the amount of food eaten will reduce a fever, says an Alberta release.

In fact, a fever is a helpful means used by our own immune system in order to fight off the viral infection. If we stop eating, the immune system doesn't work as well and all of the nutrients mentioned above, as well as many others, are important to the immune system.

The bottom line is "the key to good health is eating a well-planned balanced diet that focuses on variety", concludes Field.