Gingivitis is an irritation of the gums. It is usually caused by bacterial plaque that accumulates in the small gaps between the gums and the teeth and by calculus (tartar) that forms on the teeth. These accumulations may be tiny, even microscopic, but the bacteria in them produce foreign chemicals and toxins that cause inflammation of the gums around the teeth. This inflammation can, over the years, cause deep pockets between the teeth and gums and loss of bone around teeth—an effect otherwise known as periodontitis.

Since the bone in the jaws holds the teeth into the jaws, the loss of bone can cause teeth over the years to become loose and eventually to fall out or need to be extracted because of acute infection. Regular cleanings (correctly termed periodontal debridement, scaling or root planing) below the gum line, best accomplished professionally by a dental hygienist or dentist, disrupt this plaque biofilm and remove plaque retentive calculus (tartar) to help prevent inflammation. Once cleaned, plaque will begin to grow on the teeth within hours. However, it takes approximately 3 months for the pathogenic type of bacteria (typically gram negative anaerobes and spirochetes) to grow back into the deep pockets and restart the inflammatory process. Calculus (tartar) may start to reform within 24 hours. Ideally, scientific studies show that all people with deep periodontal pockets (greater than 5mm) should have the pockets between their teeth and gums cleaned by a dental hygienist or dentist every 3–4 months.

People with a healthy periodontium (gums, bone and ligament) or people with gingivitis only require periodontal debridement every 6 months. However, many dental professionals only recommend periodontal debridement (cleanings) every 6 months, because this has been the standard advice for decades, and because the benefits of regular periodontal debridement (cleanings) are too subtle for many patients to notice without regular education from the dental hygienist or dentist. If the inflammation in the gums becomes especially well-developed, it can invade the gums and allow tiny amounts of bacteria and bacterial toxins to enter the bloodstream. The patient may not be able to notice this, but studies suggest this can result in a generalized increase in inflammation in the body and/or cause possible long term heart problems. Periodontitis has also been linked to diabetes, arteriosclerosis, osteoporosis, pancreatic cancer and pre-term low birth weight babies.^{[citation needed]}

Sometimes, the inflammation of the gingiva can suddenly amplify, such as to cause a disease called Acute Necrotizing Ulcerative Gingitivitis (ANUG), otherwise known as "trench mouth." The aetiology of ANUG is the overgrowth of a particular type of pathogenic bacteria (fusiform-spirochete variety) but risk factors such as stress, poor nutrition and a compromised immune system can exacerbate the infection. This results in the breath being extremely bad-smelling, and the gums feeling considerable pain and degeneration of the periodontium rapidly occurs. This can be successfully treated with a 1-week course of Metronidazole antibiotic, followed by a deep cleaning of the gums by a dental hygienist or dentist and reduction of risk factors such as stress.