From 
The New York Times:
Christina Koenig found out she had breast cancer on a Friday afternoon. She was just 39 years old. On Monday, she thought she knew why the cancer had struck. “I went in and talked to a team of medical professionals who ultimately performed a lumpectomy, and I said, ‘How long has this been there?’ They said, ‘Five to ten years.’ And immediately, my mind jumped to: ‘Well, I did go through a divorce. I did have stress.’ ” Ms. Koenig, who lives in Chicago, was divorced four years before her cancer was diagnosed. Was it just a coincidence, she wondered? Now, four years later, she still wonders. So do many other women who get breast cancer. Ms. Koenig now works for Y-ME National Breast Cancer Organization, which gets 40,000 calls a year on its hot line. Over and over, she says, women ask, Did stress cause their cancer by weakening their immune system and allowing a tumor to grow? “It’s a widespread belief,” Ms. Koenig said.
And it is not restricted to women with breast cancer.
More here.