Wired has a story on research linking happiness to good health. Quote:

That’s what researchers at University College London’s Department of Epidemiology and Public Health are interested in. They have found that the functioning of certain key biological processes is improved by happiness.

“Psychosocial factors are vital to health,” said Michael Marmot, professor of epidemiology and public health at the university and director of the International Centre for Health and Society. “In people who have their basic needs met — clean water, sufficient food and shelter — a crucial determinant of health is how circumstances affect the mind. That is, psychosocial factors.”