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When it comes to what will really prevent colds and the flu, there are lots of mixed – and hysterical – messages out there. On the one hand, the Centers for Disease Control and Prevention urges everyone to get vaccinated against the flu. But a lot of alternative-medicine bloggers and the like claim the shot is simply a money-making scam pushed down our throats by Big Pharma that could cause serious harm. Then there are all the claims by supplement manufacturers that echinacea, Airborne, Emergen-C, and a host of other so-called immune boosters are the best – and most "natural" – ways to stay well or get better fast. It's enough to confuse even the healthiest guys, and it's why we now spend nearly $2 billion on such supplements each year.

Considering that this year more than 90 million Americans will catch a cold and up to 61 million will get the flu, it makes sense that we'd be susceptible to those trying to make a quick buck peddling supplements. Of course, there's no question that you're exposed to a lot of potentially sickening microbes. Every day you inhale and swallow tens of thousands of germs and play host to thousands more on your skin. In fact, you're home to more than 10 times the number of germs as you have cells in your body. But what's remarkable is that most of us still won't get sick. Some of that is because 98 percent of those germs are essentially friendly. But that other two percent, which includes the humble flu virus, would easily kill you were it not for your immune system.

Better organized and more responsive than a SEAL team, your immune system constantly patrols every corner of your body. Its first line of defense is your skin, which forms an almost impenetrable barrier, though pathogens can still enter through openings such as your nose. Even in your nostrils, however, "friendly" germs help stop invaders, while your mucous membranes trap and sweep them away. Any dangerous germs that do make it inside quickly trigger perimeter alarm systems – each pathogen has a specific protein marker on its cell membrane that identifies it as an enemy. In response, your immune system immediately notifies white blood cells called lymphocytes, which produce antibodies that hunt down the intruders. When they find the alien invaders, the white blood cells bind to the protein markers, preventing the germs from attaching to your own cells and reproducing.

That's not all. Your immune system sends reinforcements into your bloodstream to finish off the foreign bodies. These include helper T cells, which release chemicals that break down the invaders' cell walls, and their partners, phagocytes (literally "cell eaters"), which gobble them up. In addition, natural killer (NK) cells are always prowling your bloodstream to destroy any of your cells that do become infected. (NK cells also identify and kill microscopic cancers before they can spread.) And once your immune system has dealt with an enemy, it forever remembers the specific protein markers, speeding up immune response should you ever be attacked by the same germ again. Vaccines work by using a weakened, inactive form of a germ to prime your immune system to respond if need be.

When you do get sick, it's because you've come into overly close contact with germs that are either too strong or too numerous for your body to fight off – even if you're extremely healthy. That's exactly what happened when the Duke University football team ate turkey sandwiches that had been prepared by a food handler probably suffering from a Norwalk-like virus, a.k.a. the stomach flu, which 23 million Americans get each year. During a game with Florida State the next day, 43 of the Duke players became ill. But despite vomiting copiously on the sidelines, they continued to play, and the germs on their hands, uniforms, and mouthguards spread ferociously. By the end of the game, 11 Duke staff and 11 Florida State players had also become infected.

In fact, while some germs come from contaminated food or insect bites, 80 percent of all infectious diseases are spread by direct or indirect contact (from a sneeze or germs left on a phone, for example). Although you're most likely to get a cold or the flu from inhaling germs that a sick person nearby has exhaled, cold and flu germs can live on surfaces for up to three days. You're especially vulnerable if you're already sick or have been under long-term stress, failing to eat right, or not getting enough sleep – all of which, especially in combination, can reduce immune function in less than 24 hours.

Emotional stress releases the hormone cortisol, which much research shows depresses your body's elaborate defense systems. If the stress is temporary, things soon return to normal. But according to a review of 293 studies published by the American Psychological Association, chronic stress puts a stranglehold on your immune system, weakening your responses.

Lack of sleep also affects your body's ability to defend itself: According to the 'Journal of the American Medical Association,' when healthy young men were sleep-deprived the night before getting a flu shot, they developed only half the normal number of antibodies. Eating poorly (such as processed foods) and drinking too much alcohol also put a strain on your immune system because your body has to deal with chemicals and inflammatory agents. That's why avoiding things that can bust your immune system is just as important as doing things that will boost it.