If endothelial injury occurs, direct infection by pathogens such as S. aureus can result, or an uninfected platelet-fibrin thrombus may develop and become infected during transient bacteremia. The vegetation (Fig. 87-1) is the prototypic lesion at the site of infection: a mass of platelets, fibrin, and microcolonies of organisms, with scant inflammatory cells.

Figure 87-1
Vegetations (arrows) due to viridans streptococcal endocarditis involving the mitral valve.