Our understanding of the microbiome and its role in immunity, cancer initiation, and cancer progression has evolved significantly over the past century. The “germ theory of cancer” was first proposed in the early 20th century, and shortly thereafter the bacterium Helicobacter pylori and later, Fusobacterium nucleatum were implicated in the development of gastric and colorectal cancers respectively. However, with the development of reliable mouse models and affordable sequencing technologies, the most fascinating aspect of the microbiome-cancer relationship, where microbes undermine cancer immune surveillance and indirectly promote oncogenesis, has only recently been described.