Endocarditis caused by any microorganism, especially any species of streptococci or staphylococci, and less often by Haemophilus spp. or other HACEK bacteria (e.g., Actinobacillus actinomycetem comitans, Cardiobacterium hominis, Eikenella corrodens, or Kingella kingae), enteric bacteria, ricksettsiae, chlamydiae, or fungi. Traditionally, IE can be categorized as acute if the illness has a fulminant onset; catheter related if the causative microorganism gains access to the heart from an indwelling line; culture negative if echocardiograms reveal vegetations and other criteria for the disease are present, but the causative microbes have not been isolated in the laboratory; left-sided if it develops on the mitral or aortic valves; nosocomial if it occurs after 48 hr of hospitalization or an invasive surgical procedure; pacemaker-related if the disease occurs on an implanted pacemaker or cardioverter-defibrillator; prosthetic if it occurs on a surgically implanted heart valve; right-sided if it develops on the tricuspid or pulmonary valves; and subacute if it develops after several weeks or months of anorexia, low-grade fevers, and malaise. The incidence in the U.S. is about 2 to 4 cases per 100,000. Patients who are elderly or have a history of injection drug abuse, diabetes mellitus, immunosuppressing illnesses, aortic stenosis, mitral valve prolapse, or rheumatic heart disease are more likely than others to become infected.