Cancer has long been considered a genetic disease. However, accumulating evidence supports the involvement of infectious agents in the development of cancer, especially in those organs that are continuously exposed to microorganisms, such as the large intestine. Recent next-generation sequencing studies of the intestinal microbiota now offer an unprecedented view of the aetiology of sporadic colorectal cancer and have revealed that the microbiota associated with colorectal cancer contains bacterial species that differ in their temporal associations with developing tumours. Here, we propose a bacterial driver–passenger model for microbial involvement in the development of colorectal cancer and suggest that this model be incorporated into the genetic paradigm of cancer progression.