Stress will kill you. Most people know this intuitively. Stress weakens your immune system. Stress allows your body to be more easily controlled by pathogens.
A recent study compared two sets of mice. One set had a certain stress receptor cell, and one set did not. Normal mice exposed to stress had a higher rate of disease than mice exposed to stress who lacked the stress receptor cell. This held true for both psychological and physiological stress.
Mental and physical stress caused the same harmful reaction in the cells and led to disease. The cells trigger allergic reactions which cause inflammation. This can lead to heart disease, asthma, irritable bowel syndrome, and lupus. But it also weakens the immune system's ability to fight off things like common colds and flu.
Think about it. Don't you always seem to get sick at the worst time? You have a big meeting, presentation, or event coming up. Maybe you are getting ready for a vacation, stressed about flying, traveling, or making sure you get everything done before leaving.