That's what UCLA researchers discovered as they watched breast cancer tumors spread through the bodies of mice. Those tumors spread faster inside the mice that were stressed -- because they had to spend part of each day confined to a small space -- than in the mice that were not.
Stress did not appear to affect the original cancer. But once a malignancy was established, stress helped it to metastasize.
Here's what was going on inside the mice:
The cancer prompted the immune system to dispatch white blood cells called macrophages to the site of the tumor. Macrophages try to help repair damaged tissues by initiating an inflammatory response, which is how the body normally tries to heal itself. But in the case of cancer, it can backfire -- some of the compounds produced as part of the inflammatory response wind up helping the tumor cells cheat death and proliferate.
The problem with stress is that it causes the body to send more macrophages to the tumor site.
"Stress helps the cancer climb over the fence and get out into the big, wide world of the rest of the body," UCLA researcher Steven Cole, the study's lead author, said in a statement.