An adolescent with valvular disease developed low grade fever and significant weight loss, diagnosed of having subacute bacterial endocarditis. It is most commonly caused by:

Subacute endocarditis progresses to death over weeks to months with low-grade fever, night sweats, weight loss, and vague constitutional complaints. The clinical course is substantially related to the virulence of the infecting organism; Staphylococcus aureus, for example, usually produces acute disease, whereas infections by the less virulent viridans streptococci are more likely to be subacute. Ref: Ray C.G., Ryan K.J. (2010). Chapter 66. Intravascular Infections, Bacteremia, and Endotoxemia. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.