Environmental exposure to bacteria and viruses may provide oncolytic protection against cancers, and declining exposure to infections may contribute to a rising incidence of cancer.
Medical Hypothesis is one of my favorite journals; many of its articles are written by very foresighted people. This author suggests that recurrent exposure to bacteria and viruses over our lifetimes leads to constant surveillance by the immune system for these agents.
Since various infectious agents are well known to contribute to cancer (using the recent hype over the HPV vaccine for cervical cancer as an example), keeping our immune system vigilant in fighting off these agents could ward off cancer as well.