This episode covers one half of the 1966 Nobel Prize in Physiology or Medicine, which was awarded to Peyton Rous. The Nobel Assembly at the Karolinska Institute chose to give Rous the award “for his discovery of tumor-inducing viruses”. Topics include Rous’ discovery of a virus that caused cancer in chickens, how that sparked a search for cancer-causing viruses in humans, and the discovery of Epstein-Barr virus, the first virus linked to a human cancer.