The term "survival of the fittest" can take on a whole new meaning once the temperature drops, as those susceptible to bugs and flus drop like flies. So, why is it that some people get sick and some do not? Is it as simple as eating some different types of food, or not getting caught out in the cold without a jumper? Well, recent research suggests the optimising immune function in general may play more of a role in keeping us healthy this winter along with a few super flu fighters that do have research to support their use.

The immune system, like the other major systems in the body is complex. For optimal functioning in which the body naturally fights off infection, flus and bugs, it is not a matter of eating one or two specific types of food but rather making sure that you eat a number of nutrient-dense foods daily to ensure that the key nutrients are available for the immune response to work optimally.

It is the body's inability to generate bug-fighting cells, from an insufficiency of energy or key nutrients that hampers this process. This can result from both a poor diet lacking key nutrients such as vitamins C, A and E, zinc, iron and essential fats and/or a diet that is particularly low in energy; such is the case when calorie intake is low. For this reason, avoiding diets that are especially restrictive, particularly in carbs, while focusing your diet around key foods which offer these nutrients in high volumes is the first step towards taking control of your immune health this winter.

Key foods including lean red meat which offers zinc and iron; brightly coloured fresh fruit and vegetables for a vitamin boost, and wholegrains for essential fibre and B-group vitamins necessary for optimal energy production should be consumed daily.

Next and perhaps even most importantly is keeping our gut healthy. For some time researchers have known that keeping the right mix of good and bad bacteria in the gut is a key aspect of optimal immune function. Unfortunately the stress of modern life, diets high in processed foods and the regular use of antibiotics has left many of us with guts that are far from functionally optimally. Working towards better gut balance means the active inclusion of both pro and prebiotics in our daily diets. Probiotics reintroduce good bacteria to the gut and are typically found in cultured yoghurts, fermented drinks such as Yakult, fermented vegetables such as sauerkraut and kimchi and supplementary products available in pharmacies. Prebiotics on the other hand pass through the digestive tract undigested and feed good bacteria in the gut. Fibre-rich foods that act as prebiotics include asparagus, Jerusalem artichokes, leeks, onions, beans, chickpeas, lentils and supplementary fibres such as psyllium, pectin and guar gum. Aiming to include both probiotics and prebiotics in your daily diet will help to give your gut the best chance of fending off nasties this winter.

Finally we have the "special" immune foods thought to have specific colds and flu-fighting properties. Chicken soup has been a favourite remedy for hundreds of years, with many families owning a traditional soup recipe passed through the generations. And it seems that our elders had it right as there is evidence to show that chicken soup with broth made using actual chicken bones may improve immune function. Research published in the American Journal of Therapeutics found that a molecule found in chicken soup, carnosine, helped the body's immune system to fight the early stages of flu by inhibiting the migration of infected cells around the body.

Garlic is another known immune "superfood". Garlic has been used to treat bacteria, high blood pressure and infection for thousands of years and it is believed that the organosulfides (naturally occurring chemicals found in garlic and onions), along with vitamin D help to stimulate the production of the immune cells, macrophages.

Finally, both Chinese and Japanese healers have used the powers of mushrooms for centuries to boost the immune system and since mushrooms have now been shown to have some of the highest antioxidant contents of all foods it seems they are onto something. Mushrooms are particularly high in a specific antioxidant, "ergothioneine", important for transporting proteins in the bloodstream and a single serve of mushrooms also offers a quarter of an adult's daily requirement of selenium, another powerful antioxidant.