Whether you succumb to a virus or fight it off depends on how strong your immune system is. Here's how you can boost your odds as the temperatures drops.

Exercise Exercise increases the circulation of lymph (fluid providing oxygen and nutrients to cells), macrophages (cells which eat invading germs), and T-cell lymphocytes (which produce antibodies that kill viruses).

Avoid toxins Nicotine in cigarette smoke harms the mucous membranes and cilia (small hairs) in the nose that block invading organisms. Fried food is high in free radicals (harmful molecules that damage cells). Excess alcohol depletes the body of protective nutrients like zinc and vitamin C.

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Watch your diet Eat at least 5 portions of fruit and vegetables daily, and choose foods rich in these immune-enhancing nutrients:
Vitamin C — citrus fruits, red and green capsicums, kiwifruit, spinach, broccoli
Beta-carotene — orange fruit and veg (e.g. dried apricots, carrots, sweet potatoes, pumpkin), green leafy vegetables
Zinc — pumpkin seeds, seafood, lean meat, nuts, whole grains
Vitamin A — liver, dairy products, oily fish (e.g. herrings and sardines)
B-group vitamins — wholegrain bread, pasta, cereal. B vitamins particularly critical to immune health include folic acid (green leafy vegetables), and vitamin B12 (liver, fish, eggs, and brewer's yeast).

Add nutritional insurance Vitamin C won't prevent a cold, but it will ease symptoms; take in a multivitamin formula or a single dose. Sucking zinc lozenges reduces a cold's severity and duration; at the first sign, suck 1 lozenge (15-25 mg) every 2 to 3 hours. Eat yogurt with live cultures: it stimulates the production of an infection-fighting substance called gamma-interferon.

Get herbal help Echinacea stimulates production of both white blood cells and T-cells. Garlic has antibacterial and antiviral properties and is a good source of the antioxidant selenium, while shiitake mushrooms — used for centuries in Chinese medicine — contain an immune-stimulating substance called lentinan. Take a supplement or add the dried mushrooms to soups. Other useful herbs include liquorice (supports adrenal function), and elderberry (has antiviral properties).

Put up a good fight Don't forget the practical things you can do to avoid viruses and infection. Wash your hands frequently (viruses can live on skin for hours); open windows and doors (good ventilation flushes out germs), and dispose of used tissues immediately.