In 1911, Rous's seminal discovery1 that cell-free filtrates from birds with sarcoma were able to cause tumors in healthy birds laid the foundations of a new field at the crossroads of microbiology and cancer biology. The idea that viruses are causally linked to cancer was viewed with skepticism at the time and fell into oblivion for decades but reemerged later, and 55 years after his groundbreaking discovery, Rous received the 1966 Nobel Prize in Physiology or Medicine. A century after this major finding, more than 20% of cancers have been causally linked to human pathogens.2 Why an infection is sometimes controlled and on other occasions progresses to malignant tumors is still a mystery, but epigenetic changes increasingly emerge as having a causative role.