Learn how to use the most powerful tool to fend off illness and stay healthy - it's free and it's the body's best defense against disease - your immune system! This extract is taken from The Immune Advantage - How To Boost Your Immune System by Ellen Mazo, Dr Keith Berndtson and the Editors of Prevention Health Books

Have you ever known a family that has one member who catches everything going around, while her siblings seldom have even a cold? This occurs despite the fact that they share bedrooms, eat the same food and go to the same school. Long after she's become an adult, this person still has many more illnesses than her brothers and sisters or anyone else. Why is it that some people are more susceptible to the germs that assault us every day?

We all cut our hands on sharp objects, kiss people who are harbouring a flu virus, and touch surfaces that are covered with microbes. Yet, only some of us will develop an infection, come down with the flu, or be devastated by a bout of whatever is going around.

To guard us against these hazards, our body has a complex and sophisticated defence organisation: the immune system. Like any defence force, it is made up of different parts (like battalions) that stimulate each other, and work together, although each one has its own special field of activity.

If they are functioning as they should, these defences protect us against the daily bombardment of infectious micro-organisms; attacks that we usually do not even notice because our immune systems are so automatic and effective.

Our defence systems

The body's first line of defence is the innate system - the protections we are born with. They include our skin, mucous membranes, inflammatory response and secretions that contain toxic chemicals to destroy invading micro organisms. Bacteria and viruses are the most common invaders of this system.

The adaptive immune system is the body's second line of defence. It responds to an invasion of microbes by transforming whole classes of white blood cells into fighting cells, armed to attack specific invading micro organisms. This part of the immune system adapts its response to a particular invader. The process is complex, so adaptive immunity usually takes more time - days or weeks - to get under way.

The third defensive system is the lymphatic system. This is constantly at work filtering bacteria, abnormal cells and other foreign bodies from the lymph fluid.

Now that we have been introduced to the basic defence systems of our bodies, let us take a closer look at the individual parts that make up each of these general systems, beginning with the innate system.

Top 10 immune boosters

1. Put some colour on your plate
Eat 9 to 10 servings of fruits and vegetables each day. This may seem impossible to do and the average person subsists on a paltry four servings of fruits and vegetables a day. But the average person is also three kilos (seven pounds) overweight and so tired that she can barely keep her eyes open to watch her favourite weeknight drama. The good news is that meeting the nine to 10 goal is really very much easier than you think. Try the iVillage guide to portion size.

2. Take the supplements
Take a multivitamin plus vitamin E, vitamin C and calcium. The vast majority of the population does not take supplements, assuming instead that nutritional needs will be met from a balanced diet. However, even if you are generally healthy and eat well, taking a supplement may act as an insurance policy against any nutritional shortfalls. Your nutritional needs vary throughout life and are also affected by lifestyle factors such as stress, smoking and drinking alcohol. Taking supplements might be the most practical and efficient way of meeting your nutritional requirements.

3. Discover the wonders of working out
Get moving for 30 minutes a day. Just do it. It sounds so simple, yet only a fraction of the population actually exercises five times a week for 30 minutes. If you can't manage 30 minutes every day, do what you can - every little helps.

4. Enjoy the power of sleep
Get your 8 hours. At the end of the day when the chores are done, the children are in bed, and the house is still, it's finally time for you to do what you want. The TV beckons; so does the unread novel and your partner from the other room. Sleep can wait, you think.

5. Raise your emotional quotient
Pay attention to your psychological health. Few people would deny the link between mind and body: just think of how our faces pale when shocked and redden when enraged. Elsewhere in this book, we talk about the strong link between emotional well-being and immunity. Emotional honesty and self-awareness are highly important. If these are hard to come by, seeing a therapist or counsellor is a wise health investment.

6. Listen to your body
Sometimes our bodies whisper - a tiny rumble indicates hunger - and sometimes they scream - a blinding pain signals a broken bone. Often, we are too busy to heed our bodies' calls. But since our body signals offer important information about illnesses, perhaps we should start paying attention.

7. Wet your whistle
Drink eight to ten 240 ml (eight fl oz) glasses of water and one cup of tea a day. It's not included in the Recommended Dietary Allowances, but water is one of nature's most important nutrients - and one of which most people don't get nearly enough. Nothing is better than water, when it comes to beverages. It has no calories, no sugar and no caffeine. On average, women drink 4.7 cups of water-based liquids - juice, soda, coffee and so on, a day. People should be drinking twice that amount: eight to ten 240 ml (8 fl oz) glasses a day.

8. Remember, variety is the spice of life
Eat a wide variety of foods. The vast majority of the population fails to follow that advice. In one American study, researchers examined data from the second National Health and Nutrition Examination Survey (NHANES II). They found that only one-third of adults ate at least one food each day from five food groups: dairy, meat, grains, fruits and vegetables. Delving further, they found that less than three per cent ate at least two servings each day from the dairy, meat, fruit and vegetable groups and four servings from the grains group.

9. Be a fat detective
Eat the good fats, not the bad. Eating the right kind of fat is absolutely essential for health. You need to investigate which are the ones to eat and which ones to avoid.

10. Exercise kitchen care
Use safe cooking methods. Taking some simple precautions in the kitchen can have a dramatic impact on your health. Food poisoning hits the headlines when people come down with salmonella poisoning from eating at the local fast-food outlet. But still, about 20 per cent of the yearly millions of cases of food-borne illness start in the home, where you have complete control over the cooking and cleaning. Low levels of exposure to many germs help your immune system train itself to keep you well defended against infection. But exposure to high concentrations of germs (especially the more aggressive ones) can overwhelm your immune system and make you ill. If you take steps to limit your exposure to the germs in the first place, you can help protect your immune system.

The Immune Advantage - How To Boost Your Immune System by Ellen Mazo, Dr Keith Berndtson and the Editors of Prevention Health Books is published by Rodale/Pan Macmillan, £14.99