Invasive pneumococcal disease (e.g., bacteremia or meningitis) and pneumonia cause high morbidity and mortality in spite of effective antimicrobial control by antibiotics. These effects of pneumococcal disease appear due to irreversible physiologic damage caused by the bacteria during the first 5 days following onset of illness, and occur regardless of antimicrobial therapy. Vaccination offers an effective mean of further reducing the mortality and morbidity of this disease.