Gingivitis is inflammation of the gums (gingiva). You may notice that your gums look a bit red and sometimes bleed when you brush your teeth. It is caused by a build up of bacteria which is in the plaque that covers your gums and teeth. If untreated it can progress to something much worse called periodontitis where If left untreated your teeth can become loose. To prevent this from happening have good oral hygiene (regular brushing and flossing) plus visiting your dentist every 6 months for a professional clean.