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Smoking

Smoking cigarettes weakens the tiny disease-fighting hairs tucked inside nasal passages and the lungs, which trap and dispose of germs. This can leave your body more susceptible to attack. Plus, research shows that H1N1 burrows deeper into the lungs than seasonal flu, leading to infections that may be more severe than those caused by the latter.

Pascal James Imperato, MD, the dean of the School of Public Health at SUNY Downstate Medical Center, in Brooklyn, warns that prior lung damage, such as that caused by smoking, can leave you at greater risk of serious complications as well. "Chronic smokers are always much more vulnerable to severe viral infections of the respiratory type," he says. "They have damaged lungs, so they are more susceptible to coming down with illnesses and developing pneumonias following them."

Next: Hitting the gym