In 1939, E W Fish implanted bacteria into guinea pigs' jaws and reported that four zones develop. The first zone was the zone of infection, whereas the other three zones—surrounding the zone of infection—revealed immune cells or other host cells but no bacteria. Fish theorized that by removing the infectious nidus, dentists would permit recovery from the infection, and Fish's reasoning and conclusion became the basis for successful root canal treatment. Still, endodontic therapy of the era indeed posed substantial risk of failure, and fear of focal infection crucially motivated endontologists to develop new and improved technology and techniques.