Normally your body’s barriers, including your skin and the linings of your airways and gastrointestinal tract, keep invaders out. But low immunity, contact with a heavy germ load, high stress, and some surprising factors can increase your chances of getting sick.

Having fewer illnesses in childhood could affect your health as an adult. Paradoxically, people who were frequently sick with viral infections as children typically have greater immunity and are susceptible to fewer infections as adults. “Antibodies last a lifetime,” says James A. Wilde, an infectious-disease specialist and the director of the pediatric emergency department at the Medical College of Georgia, in Augusta. When your child is sick with her third cold of the season, remind yourself that it may be helpful to her in the long run.

Cold temperatures and low humidity, indoors and out, can irritate or damage your airways. Air pollution, indoor dust, and ash from fireplaces can also act as irritants, causing inflammation and making it easier for germs to enter your system, says Russell Robertson, chair of the department of family medicine at the Feinberg School of Medicine at Northwestern University, in Chicago.

Smoking can make you prone to bacterial and viral infections. “Smoking damages the linings of the nose and throat, which not only offer barrier protection but also have a coating of fine filaments, called cilia, on the surface,” says Neil Schachter, a professor of pulmonary and critical care at the Mount Sinai School of Medicine, in New York City. That’s why smokers tend to have more frequent and worse colds than nonsmokers. Exposure to secondhand smoke can also lower your natural defenses.

Stress, lack of sleep, and poor nourishment can also set you up for getting sick. Stress and fatigue can lower your resistance to infection and increase the intensity of illnesses you do get. (For more on how stress can make you sick, see Manage Your Stress.) But there may be a bit of a lag between a stressful event and when you become ill. “When you’re in a period of maximal stress, you’re releasing a lot of adrenaline, which keeps you going,” says Bruce Polsky, chief of the division of infectious diseases at St. Luke’s– Roosevelt Hospital Center, in New York City. “Once that stressful stimulus is over or removed, you crash.”

If you’re under long-term stress, you’re especially susceptible to illness, because chronically elevated stress hormones can suppress immune function and lower the activity of germ-fighting white blood cells. Researchers at Carnegie Mellon University found that severe chronic stress — from such things as conflicts with family or friends and unemployment — significantly increases a person’s risk of coming down with a cold.