Some people seem to survive cold and flu season with nary a sniffle. And yet plenty of others seem to catch cold after cold, sidelined with a perpetually stuffed-up nose and hacking cough that lasts all winter long. So what's different between these two groups?

Research shows your cold and flu vulnerability may come down to a few important—and unexpected—habits and conditions, like how much sugar you eat and how dry your nose is. Check out these six factors and learn what you can do to stay healthy.

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

Immunity buster: You don't drink enough water.
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.

Immunity buster: You drink water from a well.
Surprisingly, the cleanliness of your drinking water can play a role in whether you get sick. As many as 25 million Americans drink well water that contains more than the safe levels of arsenic determined by the U.S. Environmental Protection Agency. 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 five 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. If your well water tests high, consider switching to bottled water or investing in a remediation system that will remove the arsenic.

Immunity buster: You've got a lot of 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.

Immunity buster: Your nose is Sahara-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.

Immunity buster: You're always seriously stressed out.
It's no coincidence that you tend to catch a cold after a big work deadline. According to a report by the American Psychological Association, long-term stress weakens the responses of your immune system. In addition, if you become stressed while you have the flu, your symptoms can get worse.

Share these tips with your family and friends so you can all stay well during cold and flu season.