Etiology of infective endocarditis The causative organisms differ depending on the underlying risk factors. Fifty percent to 60% of cases of endocarditis occurring on damaged or deformed valves are caused by Streptococcus viridans, a relatively banal group of normal oral flora. By contrast, the more virulent S. aureus (common to skin) can attack deformed as well as healthy valves and is responsible for 10% to 20% of cases overall; it also is the major offender in infections occurring in intravenous drug abusers. Additional bacterial agents include enterococci and the so-called HACEK group (Haemophilus, Actinobacillus, Cardiobacterium, Eikenella, and Kingella), all commensals in the oral cavity. Ref : Harrisons 20,th edition pg 1443