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 acute care nurse practitioner practice. It is the most hemodynamically demanding of the endocarditis lessons: the patient is already admitted, already unwell, and the questions are how fast they are deteriorating, what the infection has destroyed, and when cardiac surgery needs to be involved. On the...