Infective endocarditis is an infection of the endocardial surface of the heart, most often a valve. It is uncommon, it is frequently missed early, and untreated it is close to uniformly fatal — which is why a patient with unexplained fever and a reason to have a susceptible heart deserves active suspicion rather than reassurance. What makes it distinctive is that it is an *endovascular* infection. The organism is not walled off in a tissue abscess; it is sitting on a surface washed by every liter of blood the heart moves. That single fact explains almost everything else about the disease: the persistent bacteremia, the need for prolonged intravenous therapy, the fever that does not settle, and the sudden catastrophic complications when a fragment breaks free. This lesson is written for United States family nurse practitioner practice. Its center of gravity is the primary care encounter — recognizing endocarditis in a patient who simply looks unwell, protecting the diagnosis by not reaching for oral antibiotics, deciding who cannot be worked up as an outpatient, and carrying the patient afterward through...