As many as 37% of breast cancer cases may be attributable to exposure to the bovine leukemia virus. The incidence of breast cancer continues to increase worldwide. In the United States, this amounted to a 40% increase in the incidence by the turn of the century. Presently, the main approach to preventing mortality is early detection and treatment. That’s important, but why not focus more on primary prevention—protecting people from risk factors so they don’t develop breast cancer at all? “Overall, it is estimated that 20% of all human cancers have an infectious origin.” Viruses can trigger can...