Periodontitis is a severe infection of the gums; a lighter infection of the gums is called gingivitis. The difference between gingivitis and periodontitis is mainly one of disease progression. We speak of periodontitis when the infection of the gums has progressed to reach the bone on which the teeth rest. Even though periodontitis is a disease that manifests itself with pus and foul oral smell, not everyone feels the urge to consult a dentist. The reason is that periodontitis often doesn't cause much pain.

People suffering from periodontitis may find their gums (gingiva) bleeding easily and may notice that their gums are red from inflammation. But they may consider these symptoms part of their physical constitution, just like shortsightedness, or baldness, or pimples, and not as a disease that requires the attention of a physician.

Periodontitis and gingivitis start with the plaque that forms mostly at the base of the visible part of the teeth. This plaque is caused by food debris and bacteria that grow on it. Tooth brushing removes plaque and thereby constitutes a preventive measure against periodontitis. However, some plaque is likely to remain in spite of brushing one's teeth. This plaque hardens and then is called calculus.

Because of its hardness, calculus will often result in a mechanical irritation of the gums. Additionally, calculus provides an area on which more bacteria easily grow. Like coral reefs, plaque and calculus will, if not removed, continue to grow and form deep pockets at the base of the teeth that will sooner or later fill with pus.

While plaque can be avoided to some extend by brushing teeth, calculus can only be removed with sharper objects. Obviously, in order not to carry more pathogens into the open wounds, these tools must be professionally sterilized. This requirement, and the danger of accidental deep wounds, is the main reason why there are no public health campaigns to make use of such tools oneself. Instead, in advanced countries like the US and Western European nations, there are public health campaigns that urge the public to visit a dentist regularly for calculus removal (professional cleaning).

In cases of light gingivitis, the gums may heal all by themselves after pockets of calculus are removed. In more advanced cases of gingivitis, or in cases of periodontal disease (periodontitis), or in cases where gingivitis is accompanied by some systemic illness, a dentist will likely prescribe a broad-spectrum antibiotic to support the body in getting rid of the pathogens that caused the gum infection.

Left untreated, gingivitis will, if a patient lives long enough, develop into periodontitis, which will lead to teeth falling out, as well as possible systemic complications. Periodontitis also makes dental caries much more likely, which also leads to the loss of teeth.