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 adult-gerontology primary care nurse practitioner practice. Its emphasis is the complex adult — the patient with a prosthetic valve, an implanted device, a dialysis catheter, reduced renal function and eight other medications — in whom endocarditis presents quietly and in whom treatment itself carries real risk. On...