Infective endocarditis is an infection of the inner surface of the heart (endocardium), usually the valves. Signs and symptoms may include fever, small areas of bleeding into the skin, heart murmur, feeling tired, and low red blood cell count. Complications may include backward blood flow in the heart, heart failure – the heart struggling to pump a sufficient amount of blood to meet the body's needs, abnormal electrical conduction in the heart, stroke, and kidney failure.

The cause is typically a bacterial infection and less commonly a fungal infection. Risk factors include valvular heart disease, including rheumatic disease, congenital heart disease, artificial valves, hemodialysis, intravenous drug use, and electronic pacemakers. The bacteria most commonly involved are streptococci or staphylococci. Diagnosis is suspected based on symptoms and supported by blood cultures or ultrasound of the heart. There is also a noninfective form of endocarditis.

The usefulness of antibiotics following dental procedures for prevention is unclear. Some recommend them for people at high risk. Treatment is generally with intravenous antibiotics. The choice of antibiotics is based on the results of blood cultures. Occasionally heart surgery is required. The number of people affected is about 5 per 100,000 per year. Rates, however, vary between regions of the world. Infective endocarditis occurs in males more often than in females. The risk of death among those infected is about 25%. Without treatment, it is almost universally fatal. Improved diagnosis and treatment options have significantly enhanced the life expectancy of patients with infective endocarditis, particularly with congenital heart disease.

Infective endocarditis is divided into three categories of acute, subacute, and chronic based on the duration of symptoms. Acute infective endocarditis refers to the presence of signs and symptoms of infective endocarditis that are present for days up to six weeks. If these signs and symptoms persist for more than six weeks but less than three months, this is subacute infective endocarditis. Chronic infective endocarditis refers to the presence of such signs and symptoms when they persist for more than three months.

This classification is now discouraged because the ascribed associations (in terms of organism and prognosis) were not strong enough to be relied upon clinically. The terms short incubation (meaning less than about six weeks) and long incubation (greater than about six weeks) are preferred.

Infective endocarditis may also be classified as culture-positive or culture-negative. By far the most common cause of "culture-negative" endocarditis is prior administration of antibiotics and can occur in up to 31% of cases.

Sometimes microorganisms can take a longer period to grow in the culture media, for example Cutibacterium spp. and the HACEK bacteria group. Some organisms are said to be fastidious because they have demanding growth requirements. Some examples include pathogens like Aspergillus species, Brucella species, Coxiella burnetii, Chlamydia species. Due to delay in growth and identification in these cases, patients may be erroneously classified as "culture-negative" endocarditis.

Endocarditis can also be classified by the side of the heart affected: