What is plaque and why is it bad? (from ADA.org)

Plaque is a clear film of bacteria consistently sticking to the teeth. As it accumulates, a hard layer of tartar which only the dentist can remove, forms between the teeth and near the gumline. Bacterial plaque contains toxic chemicals that irritate the gums. If it is not properly removed, this bacteria can cause gum disease which affects the underlying bone and blood vessels around the tooth. Due to bacteria entering the blood, gum disease is linked to other health problems including heart disease, stroke, pneumonia, and pregnancy complications. Removing plaque by brushing and flossing on a daily basis twice a day, along with regular visits to the dentist is the first step in preventing gum disease and other health problems. Once gums begin to hurt, it may be too late. By then it is in its advanced stage. Visiting your dentist on a regular basis helps prevent gum disease and improves overall health.