From Nature:
Stress has acquired a bad image as a contributor to disease, but a little stress may be no bad thing. Mice raised in a complex environment providing social interactions, opportunities to learn and increased physical activity are less likely to get cancer, and better at fighting it when they do, a new study suggests. A mild boost in stress hormones seems to be what keeps the cancer at bay by switching on a molecular pathway that restrains tumour growth.
Researchers from the United States and New Zealand injected mice with melanoma cells — the deadliest form of skin cancer. After six weeks, mice raised in an enriched environment — extra-large cages housing 20 individuals with running wheels and other toys — had tumours that were almost 80% smaller than those in mice raised in standard housing — five animals to a cage with no additional stimulation. Whereas all the normally housed mice developed tumours, 17% of the mice from the enriched environment developed no tumours at all. Tests in mice with colon cancer showed the same effect.
More here.