O, The Oprah Magazine (January 2010) considers a Stanford study about how stress can boost immune system:

A Stanford University study found that acute stress may protect against a common type of skin cancer. “Quick bursts of stress seem to direct the body’s ‘soldiers’ [cancer-fighting immune cells] to ‘battlefields’ [skin and lymph nodes,] increasing their ability to fight damage,” says lead author Firdaus Dhabhar, PhD. So while it may seem counterintuitive, try to get your stress hormones flowing for short periods (think playing sports, not hosting your mother-in-law for two weeks.)