Happiness does not heal, but happiness protects against falling ill. As a result, happy people live longer. The size of the effect on longevity is comparable to that of smoking or not
While I agree with what some commenters said about the deficiencies of such studies I really think that there is something in this. (....which is interesting given my increasingly morose outlook on life at the moment!)
Anyway, here is a study that says that women exposed to negative life events at greater risk of breast cancer:
Happiness and optimism may play a role against breast cancer while adverse life events can increase the risk of developing the disease, according to a study by Professor Ronit Peled, at Ben-Gurion University of the Negev in Israel.
The BBC also noted this one
There is of course another side to this.