Smoking — There are dozens of toxic chemicals in cigarettes and cigars. Whether it's first- or second-hand smoke, exposure to these toxins weakens the immune system and causes inflammation.

Don't smoke or quit. Immune activity improves within 30 days of quitting.

Drinking in excess (defined as more than 1 drink a day for women and 1-2 a day for men) impairs the function of white blood cells.

Commit to moderation if you choose to drink.

Chronic stress — The continual release of stress hormones (to battle what the body believes is trouble) means chronic exposure to these strong substances. That may accelerate the aging of the immune system and decrease the strength and power of immune cells.

Recognize when and how often you feel anxious, worried and worn out. Promise yourself (and keep it) that you'll take at least 15 minutes a day to take care of you. Know what makes you happy, what eases your mind and give yourself permission to use that time to keep yourself well.

Couch-Potato Lifestyle — Studies have found that people who get little physical activity tend to be sick more often than those who are "up and at 'em." Exercise stimulates the activity of certain immune cells. Not moving much may depress their function and add even more immune troubles: excess weight.

Work up to getting 30 minutes of daily non-stop physical activity that leaves you winded, but still able to talk. Check with your health care provider for specific guidance.

Age — Over time, exposure to harmful substances can wear down the immune system. Some researchers argue that age alone does not necessarily determine the strength of the immune system. The number of immune cells we produce may not necessarily decrease, but their responsiveness may change. Good lifestyle habits, including a diet filled with healthful foods, greatly determines how much aging plays a role in our immune power.

To a certain extent, age is a matter of mind over matter. What makes you "young" is your outlook on life and how you choose to take care of you. Choose good ways for a good life.

Depression — Many studies have found overall outlook and mood affect the immune system. Difficulty being happy and optimistic can hold the immune system back.

Take stock in how you feel daily. If it's not hopeful and happy, talk with a health care professional to see how best to move forward.

Antibiotics are only beneficial for bacteria-related infections, not virus-related ones. Antibiotics kill the good bacteria as well as the bad, including the good bacteria in the intestines. That makes the intestines an easy target for harmful substances to get in and cause problems.

If antibiotics are prescribed to you, talk with your doctor about whether they truly are necessary for what ails you. If yes, then take them exactly as prescribed to ensure they do what they were intended to do.

Eating Disorders — These conditions often put the body in a malnourished state (calories, protein, vitamins, minerals, etc.) That significantly weakens the immune system and slows its response to illness and harmful germs.

Talk with a health care provider if there are significant changes in your eating habits and weight or if someone you know is experiencing those changes.

Overeating — Research suggests overeating may trigger a signal in the brain that causes immune cells to go out and attack what the body perceives as invaders. Healthy cells may be damaged instead.

Watch for changes in how your clothes fit. If they're getting snug–pay attention to your weight change sooner rather than later. Talk with a dietician about how to eat well and bump up the activity portion of your life. The quicker you jump on it, the easier it will be.

Too much body fat stimulates inflammation and the creation of substances that can weaken the immune system.

Keeping weight in line is no easy task. The three most influential ways to lower body fat are: better eating habits, choosing mostly healthful foods and getting and staying active. There are several ways to achieve each of these. Find the plan that works with your lifestyle and interests.

Yo-Yo Dieting — Continual fluctuations in your weight may decrease the strength or potency of natural killer cells–cells in the immune system that bind to harmful bugs and release chemicals that can kill invaders.

Take time to figure out what kind of weight plan is best for you and your interests and motivation. Talk with a registered dietitian to create a plan that is personalized to you and your needs. It's your best bet for lasting success.

The bacteria and virus-fighting capabilities of white blood cells may be weakened for several hours after these foods are eaten.

Eating plenty of fruits, vegetables, whole grains and beans can be very tasty and highly satisfying way to nudge out room for immune sabotaging foods. Fill half your plate with fruits and vegetables. Think quality of treats vs. quantity. Eat slow and savor small indulgences. Scrap the overly processed snacks and sweets.