As a companion piece to Desi's post:

Stress does not cause breast cancer (or any type of cancer), and it does not cause RSD. It is merely a contributing factor in just about any physical disease, not a cause in and of itself.

Once upon a time, doctors thought stress caused ulcers, but it turns out it was a bacterial infection all along. Stress is blamed for heart attacks, but the truth is that it merely plays a role among other factors--heredity, diet and age are the biggest culprits.

Stress can certainly be harmful, and chronic stress can run down a person to the point where these other factors come into more prominent play. It doesn't cause disease, it just sets the stage for disease to happen more easily.

-Betsy