Streptococcus pneumoniae are lancet-shaped, gram-positive, facultative anaerobic bacteria with 100 known serotypes. Most S. pneumoniae serotypes can cause disease, but only a minority of serotypes produce the majority of pneumococcal infections.
Pneumococci are common inhabitants of the respiratory tract. The bacteria may be isolated from the nasopharynx of 5–90% of healthy persons, depending on the population and setting:
- 5–10% of adults without children are carriers
- 20–60% of school-aged children may be carriers
- 50–60% of service personnel on military installations may be carriers
The duration of carriage varies and is generally longer in children than adults. In addition, researchers do not clearly understand the relationship of carriage to the development of natural immunity.