Current research shows that stress can suppress your immune system, perhaps opening the door to a number of infectious diseases. “Stress doesn’t make you sick,” says virologist Ronald Glaser. “But it does increase your risk of being sick because of what it does to your immune system.” There is particularly compelling evidence linking stress to colds, the flu, and herpes. Although we are continually exposed to such viruses, our immune system normally fights them off. But some experts say that when a person is under emotional distress, these defenses can fail.

The biological mechanisms involved are not yet fully understood, but some theorize that the hormones that gear you up for action when you are under stress can hamper your immune functioning as they surge through the bloodstream. Usually, this is not a cause for concern, since these hormones are only on a temporary mission. Nevertheless, some say that if a person faces stress that is ongoing and intense, his immune system may be compromised to the extent that he becomes susceptible to illness.