Patients who are on steroid treatments, chemotherapy, or other medications that suppress the immune system are also prone to infection. People with diabetes are at higher risk for flu. Certain genetic disorders predispose people with these problems to respiratory infections. Much evidence suggests that stress increases ones susceptibility to a cold. In one study, people with high stress levels averaged 2.7 upper respiratory infections during a six-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.
Although long-term effects of regular exercise are known to improve health, the immediate effect of exercise on the immune system is uncertain. High-intensity or endurance exercises might actually suppress the immune system while they are performed. Some highly trained athletes, for instance, report being susceptible to colds after strenuous events. A recent study suggested that in people who already have colds, exercise has no effect on the illness' severity or duration of the infection. People should avoid strenuous physical activity when they have high fevers, muscle aches, productive coughs, or widespread viral illnesses.