Stress and Your Immune System
While short-term stress fires up your immune system, long-term stress triggers a cascade of anti-immune reactions. High levels of the stress hormone cortisol cause the body to cut back on what it considers non-essential systems like immunity. As a result, any bacteria you encounter are more likely to take hold.
In a study at the University of Pittsburgh 276 volunteers were asked about stresses in their lives, then exposed to cold virus. Those who'd reported chronic stress were more than twice as likely to succumb to the virus.