1. You have a sweet tooth. Eating too much sugar doesn't just pack on pounds... A study published in the American Journal of Clinical Nutrition found that eating 100 g of sugar (think three cans of soda) significantly hampered the ability of white blood cells to kill bacteria for up to 5 hours afterward.

2. You don't drink enough. There's a reason moms and doctors always push fluids on you when you're sick. Your body needs plenty of water to flush out toxins--and yes, coffee and tea are acceptable sources. How much fluid you should drink daily varies from person to person. You're drinking the right amount if your urine is pale yellow.

3. You've got weight to lose. You know excess weight is unhealthy for your heart, brain and other organs. But it's also bad for your immune system. In fact, those who become most seriously ill with swine flu tend to share the same characteristic: a body mass index over 40, meaning they are morbidly obese. Excess weight can cause hormonal imbalances and inflammation that impairs the immune system's ability to fight infection.

4. You drink well water. Surprisingly, the cleanliness of your drinking water can play a role in whether or not you get sick. As many as 25 million Americans drink well water that contains more than the safe levels of arsenic determined by the EPA. Arsenic has been linked to several different cancers, and affects the immune response to swine flu as well. When researchers from Dartmouth Medical School inoculated two groups of mice with the H1N1 virus, the group that had spent 5 weeks drinking arsenic-tainted water developed suppressed immune systems, and many died. The mice that didn't drink the water got the flu but recovered completely.

5. Your nose is dry. As uncomfortable as it may be, a runny nose is actually a good defense against colds and the flu. Sounds gross, but mucus traps viruses and clears them from the body. If your nasal passages are too dry, germ invaders have an easier time. If the dryness is a temporary problem, irrigate your nasal passages with a squeeze bottle or neti pot of saline solution. A humidifier can also help. If dryness is chronic, see your doctor to determine the underlying cause