Bacteria were first demonstrated scientifically in the diseased dental pulp by Miller (1894) .Researchers presented the idea that the dissemination of microorganisms, or their associated toxins, throughout the body from a focus of circumscribed infection at a particular tissue site could initiate or exacerbate systemic disease or damage a distant tissue ( Daland 1916, Gilmer 1916, Hunter 1921, Thompson 1925).This concept became known as the Focal Infection Theory.