The most common cause of gingivitis is poor oral hygiene. Gingivitis can occur due to reasons other than poor oral hygiene and these are puberty, pregnancy, diabetes, leukemia, malnutrition, scurvy, pellagra and poorly fitted dentures, teeth caps, tooth filling and dental plate.
Gingivitis normally starts with plaque that is a sticky and invisible film of bacteria. Your food contains starches and sugars and they interact with bacteria in your mouth to form plaque. Although you can remove plaque by brushing, it forms again within a day. If you allow plaque to stay on your teeth more number of days they harden and becomes tartar/ calculus. Tartar is very hard and cannot be cleaned by brushing and flossing and requires professional cleaning by dentist. If the plaque and tartar continue in your teeth for a longer period, your gums swell and start bleeding.