For gingivitis to develop, plaque must accumulate in the areas between the teeth. This plaque contains large numbers of bacteria thought to be responsible for gingivitis. But it is not simply plaque that causes gingivitis. Almost everyone has plaque on their teeth, but gingivitis is far less common.
It is usually necessary for the person to have an underlying illness or take a particular medication that renders their immune system susceptible to gingivitis. For example, people with diabetes, Addison's disease, HIV, and other immune system diseases have weaker ability to fight bacteria invading the gums. People with Sjögren's syndrome have chronic dryness of the mouth that predisposes them to develop gingivitis.
Sometimes hormonal changes in the body during pregnancy, puberty, and steroid therapy leave the gums vulnerable to bacterial infection.
A number of medications used for seizures, high blood pressure, and organ transplants can suppress the immune system and change the structure of the gums enough to permit bacterial infection