Pneumococcal disease refers to a variety of infections caused by the bacterium Streptococcus pneumoniae or S. pneumoniae. S. pneumoniae is commonly referred to as the pneumococcus (or, for two or more organisms, as pneumococci). The most common types of pneumococcal infections and the approximate number of yearly cases are given in Table 1. Acute infection due to the pneumococcus is the most common cause of serious infection in Americans. Over seven million pneumococcal infections occur in the United States annually. About 40,000 Americans die annually from pneumococcal infections, a total which makes the pneumococcus the most lethal bacterium in the United States. Table 2 shows the main causes of death in Americans due to infections for which immunizations are recommended. This table clearly shows the importance of pneumococcal infections. For unclear reasons the pneumococcus causes serious illness more frequently in blacks and Native Americans. Figure 1 shows the risk from pneumococcal bacteremia in Monroe county in upstate New York. The term pneumococcal bacteremia refers to a serious pneumococcal infection which occurs when the bacteria invade the blood stream. The data are given as cases of bacteremia per 100,000 population. At all age groups blacks are two to four times more likely to develop pneumococcal bacteremia. Native Americans also suffer from high rates of serious pneumococcal disease. They have the highest rates of invasive pneumococcal disease of any racial/ethnic group studied (Table 3). The term invasive pneumococcal disease refers to serious pneumococcal infections which occur when the bacteria invade a body site which is normally sterile, such as the blood stream or the cerebrospinal fluid.