Periodontal disease has an infectious aetiology. Its extent and severity depends on the interaction between the bacteria and the host response. The host response to pathogenic microorganisms involves a complex network of cell and humoral components of the immune system, which interact with each other. The progression of periodontitis and the rate of progression of periodontitis cannot be explained solely by the presence of a microbiota. Hence, an understanding of the factors that influence this susceptibility may be essential to determine periodontal disease activity. Environmental risk...