Bleeding gums are normally caused by infections of the tissues around the teeth.
Plaque which live in the mouth cause irritation. Swelling and bleeding when the teeth are brushed or flossed characterize the infection.
Plaque can form on any tooth surface, but the accumulation is heaviest between the teeth and along the gum line, areas that are not self-cleaning.
If the plaque is allowed to harden and accumulate, a more serious condition, periodontitis, may develop. Inflamed gums are thought to be the cause of more teeth lost than through tooth decay.