This episode covers the 1905 Nobel Prize in Physiology or Medicine, which was awarded to Robert Koch. The Nobel Assembly chose to give Koch the award “for his investigations and discoveries in relation to tuberculosis”. Koch's key contribution was the development of a set of criteria for experimentally proving that a particular microorganism causes disease. These criteria are known as Koch’s Postulates. Topics include discussion of each of the postulates, Koch's application of the postulates to tuberculosis, and some cases where the postulates break down.