In a London hospital a man's cancer is held at bay by injecting it with bacteria. In an American laboratory a mouse thrives with a rat's heart beating inside it. These experiments are part of a revolution in modern medicine. The movement's chief proponent is Polly Matzinger, who tells Horizon the story of her unusual life, and the inspiration behind her idea for the Danger Model - the result of her belief that the body's immune system responds to danger and not simply foreign bodies.
