I heard Harald zur Hausen hypothesize that a cow virus might be responsible for most cases of colon cancer.
And why should anyone pay attention to what Harald Zur Hausen thinks? Well, he won a Nobel Prize in 2008 for proving that most cases of cervical cancer are caused by a few strains of Human Papilloma Virus (HPV). Nor is HPV the only viral cause of cancer. Chronic infection with certain hepatitis viruses, for example, is a major cause of liver cancer.