Endocarditis is an inflammation of the inner layer of the heart, the endocardium. It usually involves the heart valves. Other structures that may be involved include the interventricular septum, the chordae tendineae, the mural endocardium, or the surfaces of intracardiac devices. Endocarditis is characterized by lesions, known as vegetations, which is a mass of platelets, fibrin, microcolonies of microorganisms, and scant inflammatory cells. In the subacute form of infective endocarditis, the vegetation may also include a center of granulomatous tissue, which may fibrose or calcify.

There are several ways to classify endocarditis. The simplest classification is based on cause: either infective or non-infective, depending on whether a microorganism is the source of the inflammation or not. Regardless, the diagnosis of endocarditis is based on clinical features, investigations such as an echocardiogram, and blood cultures demonstrating the presence of endocarditis-causing microorganisms.

Signs and symptoms include fever, chills, sweating, malaise, weakness, anorexia, weight loss, splenomegaly, flu-like feeling, cardiac murmur, heart failure, petechia (red spots on the skin), Osler's nodes (subcutaneous nodules found on hands and feet), Janeway lesions (nodular lesions on palms and soles), and Roth's spots (retinal hemorrhages).