http://news.oneindia.in/2010/09/17/stressspeeds-up-breast-cancer-progression-inmice.html

Quote directly from the article:

“Scientists recently found that chronic stress acts as a sort of fertilizer that feeds breast cancer progression, significantly accelerating the spread of disease in animal models.

Researchers at UCLA's Jonsson Comprehensive Cancer Center have discovered that stress is biologically reprogramming the immune cells that are trying to fight the cancer, transforming them instead from soldiers protecting the body against disease into aiders and abettors.”