We know some cancers can be caused by viruses. And we know a few cancers that act like viruses in dogs and Tasmanian devils, and some rare cases in humans.
We only figured out that ulcers are bacterial in origin within the lifetimes of many HN readers, and there are signs that other GI issues may be bacterial or viral (or bacteria-targeting viral) as well.
Maybe we need to start culturing and DNA testing cancers.