There is a symbiotic relationship between the gastrointestinal microflora and the human host. Commensal bacteria provide essential nutrients to the epithelium and promote healthy immune responses in the gut. Commensal bacteria such as Escherichia coli can, however, transform into pathogens when they acquire genetic material encoding virulence factors such as adhesins, enterotoxins, invasins and cytotoxins. Enterovirulent organisms 'communicate' with the host by a variety of diverse mechanisms; these underpin the pathogenic processes that are essential for the expression of diarrhoeal disease. Many of these mechanisms involve the activation of signal transduction pathways in epithelial cells. Classical pathways include activation of adenylate or guanylate cyclases to produce chloride secretion, and subversion of cytoskeletal functions to effect intimate attachment with or without invasion of epithelial cells. Other systems are also involved, including inflammatory cells and local neuroendocrine networks. Understanding the complex interactions between the human gastrointestinal tract and the commensals and pathogens which it encounters will hopefully help us to exploit further the beneficial effects of the 'marriage' and to find new ways of preventing and treating microbial disease of the intestine which occurs when the symbiotic arrangement breaks down.