Understanding the etiology and pathogenesis of periodontal diseases and their chronic, inflammatory and infectious nature necessitates recognizing the possibility that these infections may have effects somewhere in the body. The concept that oral diseases and systemic diseases influence each other goes back to the theoryof “focal infection”. In 1891, Miller published his theory regarding focal infection in which he indicated that microorganisms and their products are able to access parts of the body that are adjacent to or distant from the mouth. The proponents of this concept assume that microorganisms present in dental plaque and their metabolic products may enter the bloodstream, thereby causing many systemic diseases and sometimes resulting in degenerative conditions. Associations have been reported between periodontal disease and cardiovascular disease (CVD), stroke, diabetes, preterm low birth-weight babies, respiratory infections and rheumatoid arthritis (RA).2