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Francis Peyton Rous discovers a virus that can cause cancer in chickens. In 1909, a farmer brought Rous a hen that had a breast tumor. Rous performed an autopsy, extracted tumor cells and injected them into other hens, which subsequently developed tumors. This is the first experimental proof of an infectious etiologic agent of cancer. Rous is awarded the Nobel Prize in Medicine or Physiology in 1966.