## Infective Endocarditis: Core Concepts Infective endocarditis (IE) is a bacterial or fungal infection of the cardiac valves and endocardium, characterized by the formation of vegetations — complex masses composed of fibrin, platelets, inflammatory cells, and microorganisms — that adhere to damaged or turbulent-flow valve surfaces. Understanding who gets IE, what organism is involved, and which valve is affected drives all clinical decision-making. Three highest-risk populations: - IV drug users (IVDU): Inject organisms directly into venous circulation → right-sided IE, predominantly tricuspid valve. Classic organism: *Staphylococcus aureus* (MRSA or MSSA). Present with pulmonary emboli symptoms (pleuritic chest pain, haemoptysis) rather than systemic emboli. - Prosthetic valve recipients: Early prosthetic valve IE (<60 days post-op) = *Staphylococcus epidermidis* or *S. aureus* from surgical contamination. Late prosthetic IE (>60 days) = community-acquired organisms including *S. aureus* and viridans streptococci. - Structural heart disease: Congenital defects, rheumatic heart disease, and degenerative valve disease create turbulent flow and endothelial damage — the foundation on which vegetations form. Causative organisms by frequency: - *S. aureus*: most common overall; causes acute, destructive IE - Viridans group...