People have been saying there is a correlation between stress and illness for years, but I'd never read thing about the possible causation until yesterday's NYT:
Chronic stress and illness are intertwined. It is well known that psychological stress raises the risk of heart disease, cold and flu, and even allergies.
But how does one lead to the other? New research suggests that the hormone cortisol plays a critical role. Released in greater amounts in times of stress, this hormone provides the body with a burst of energy. It also helps suppress the body’s immune response to infections like the flu, keeping inflammation responses like coughing, sneezing and fevers in check... Chronic stress may raise the risk of sickness by fostering resistance to cortisol.
So take a day off! Call it a pre-sick day.