Most often, endocarditis is caused by bacteria. Colonies of microorganisms, most commonly streptococcus or staphylococcus, form wartlike growths on the surface (endocardium) of the heart valves. These clusters of bacteria also contain blood cells and other material such as fibrin (a protein instrumental in blood clotting). Endocarditis is dangerous in two ways. Most commonly, the infection will eventually destroy the heart valves, leading to heart failure. In some cases, a cluster of bacteria and the surrounding debris, known as an embolus, can break off from the heart valve and travel through the bloodstream to other parts of the body. Depending on where the embolus travels to, it can cause heart attack, stroke, infection in the spleen or liver, and cut-off of blood supply to the arm or leg. Streptococcus and staphylococcus are the most common bacteria that cause endocarditis, however heart valve infection can be caused by many other bacteria and fungi. Viruses are not known to cause endocarditis.

Possible warning signs include weakness, fatigue, a slight fever, aching joints, and appearance of new, tiny dotlike areas on the back, chest, fingers, and toes. There is often a heart murmur indicating an abnormal valve or other heart defect. An eye exam may show small hemorrhages in the mucous membranes. In some cases, the disease comes on suddenly, with high fever, shaking, chills, and rapid destruction of the involved heart valve. Blood tests and cultures should be performed to identify the invading microorganism. An echocardiogram is used to check heart valves for bacterial growth and damage.

Successful outcomes for bacterial endocarditis are more likely when the condition is detected early and treated appropriately. Frequently, however, the disease smolders undetected until it has an opportunity to cause serious damage to one or more heart valves. The best outcome in endocarditis requires specialists in heart medicine, heart surgery and infectious disease. Once the cause of the infection has been determined, aggressive treatment with the most effective antibiotics must begin immediately. This always requires hospitalization, at least initially. Antibiotic therapy may continue for a month or more. If a heart valve has been seriously damaged or there is evidence of an embolus, surgery to replace it with an artificial valve may be needed.