Periodontitis, or gum disease, affects millions of people each year. Although it is associated with a defined microbial composition found on the surface of the tooth and tooth root, the contribution of bacteria to disease progression is poorly understood. Commensal bacteria probably induce a protective response that prevents the host from developing disease. However, several bacterial species found in plaque (the 'red-complex' bacteria: Porphyromonas gingivalis, Tannerella forsythia and Treponema denticola) use various mechanisms to interfere with host defence mechanisms. Furthermore, disease may result from 'community-based' attack on the host. Here, I describe the interaction of the host immune system with the oral bacteria in healthy states and in diseased states.