These papers provide an overview to psychoneuroimmunology (PNI). The principle they demonstrate is that negative emotional states, such as chronic stress, depression or anxiety, have a negative effect on the immune system. This effect increases the risk of cardiovascular disease, diabetes, Alzheimer's, cancer and other illnesses. Below is a great quick overview.

Maes, M. (2001). Psychological stress and the inflammatory response system. Clinical Science, 101, 193-194.

See also
Psychoneuroimmunology Home Page
UppityScienceChick.com
Evidence-based, With Attitude
Book Review
The Psychoneuroimmunology of Chronic Disease