Periodontal diseases or gum diseases range from the relatively mild form called gingivitis to destructive forms called periodontitis. Gingivitis is caused by an accumulation of dental plaque at the gum margin and is a common inflammatory condition. However, in susceptible people and under certain conditions the interaction between the immune system and the polymicrobial composition of the plaque changes so that certain pathogenic bacteria emerge which dysregulate the immune system. This leads to destruction of the tooth’s supporting tissue, including the bone of the tooth socket. The resulting loss of tooth support produces a pocket under the gum. This is called periodontitis (or severe periodontal disease). A key bacterium in this progressive disease is Porphyromonas gingivalis, and it is found at the base of the periodontal pocket. The emergence of P. gingivalis leads to further loss of support of the tooth to produce a deep periodontal pocket with substantial loss of bone; and this is called severe periodontitis (as shown in the images).