People with diabetes are at higher risk for flu.
Certain genetic disorders predispose people with these problems to respiratory infections. They include sickle-cell disease, cystic fibrosis (which causes mucus abnormalities), and Kartagener syndrome (which results in malfunctioning cilia).
People under Stress
Much evidence suggests that stress increases one's susceptibility to a cold. In one study, people with high stress levels averaged 2.7 upper respiratory infections during a 6-month period and those reporting low stress averaged 1.5 infections. Stress appears to increase the risk for a cold regardless of lifestyle or other health habits. And once a person catches a cold or flu, stress can make symptoms worse.
It is not clear why these events occur. Some experts believe that stress alters specific immune factors, which cause inflammation in the airways. One 2001 study reported that the only people who got sick after experiencing short stress were those whose body responded to stress with high levels of cortisol, a stress hormone, coupled with a low immune response.
Excessive Exercise
In people who already have colds, exercise has no effect on the illness' severity or duration of the infection. High-intensity or endurance exercises, however, appear to suppress the immune system while they are being performed. Some highly trained athletes, for instance, report being susceptible to colds after strenuous events. People should avoid strenuous physical activity when they have high fevers or widespread viral illnesses. Note: Very low fat diets appear to worsen this dampening effect on the immune system. A higher fat-diet may help redress this imbalance (omega-3 fatty acids, found in fish and canola oil are preferred). Whether carbohydrate loading provides much additional value is not clear.
Seasonal Incidence
Colds and flus occur predominantly in the winter. Flu season typically starts in October and lasts into mid March. In 1999, for example, doctors' office visits significantly increased beginning in December and influenza activity peaked during the first 2 weeks in February.
The reasons for this seasonal bias are not due to the cold itself, but to other factors. Certainly, flus and colds are more like to be transmitted in winter because people spend more time indoors and are exposed to higher concentrations of airborne viruses. Dry winter weather also dries up nasal passages, making them more susceptible to viruses. Some experts theorize that the high rates of viral infections in winter may be due to certain immune factors, which react to light and dark and affect a person's susceptibility to viruses.
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