S. pneumoniae is worldwide in distribution. It is the majorcause of community-acquired bacterial pneumonia. S. pneu-moniaeserotypes 6, 14, 18, 19, and 23 are usually associatedwith infections in children, whereas serotypes 1, 3, 4, 7, 8, and 12 predominate in infections in adults. S. pneumoniae sero-types 6, 14, 18, and 23 cause 60–80% of lower respiratory tract infections.
Pneumococcal infection accounts for more deaths than any other vaccine preventable disease. Children between 6 months and 4 years of age and adults over 60 years of age are most commonly at risk for pneumococcal infection. In developing countries like India, the incidence of pneumococcal diseases in children is many times higher than that in the developed countries.
S. pneumoniae is a normal inhabitant of throat and nasophar-ynx. Nasopharyngeal colonization occurs in approximately 5–75% of the population. Colonization is more common in children than in adults. The colonization occurs at about 6 months of age.
Pneumococci are strict parasites, and cause infection only under specific predisposing conditions like prior viral or other infections, aspiration, immune suppression, anatomi-cal deformity, etc. S. pneumoniae infection occurs exclusively in human beings, and no animal reservoir is found in nature. Respiratory and pharyngeal secretions of carriers and patients are the sources of infection. Horizontal transmission requires close person-to-person contact, hence overcrowding facili-tates spread of infection. Infection is acquired by inhalation of droplets nuclei and by coming in contact with contaminated fomites.