Paul Ehrlich’s pursuit of drugs to combat infectious diseases led him to the notion of a ‘magic bullet’ – something that could kill microbes such as bacteria without any harm befalling the infected individual. He also came up with the word ‘chemotherapy’ to mean the use of chemicals to treat disease. Having chronicled the rise in the profile of microbiota in cancer, it might be informative, at the outset of this chapter, to refer to a striking demonstration of the challenge presented by bacteria because it has a strong parallel with cancer.