The human nasopharynx is the only natural reservoir of S. pneumoniae. Carriage rates in young children range up to 75 percent.1 Transmission of S. pneumoniae is by contact with respiratory droplets, and although nasopharyngeal colonisation precedes disease, most who are colonised do not develop invasive disease. The nasopharynx is a source of spread between individuals, and reduction of S. pneumoniae invasive serotypes in children by vaccination results in less transmission to, and disease in, adults. Invasive pneumococcal disease (IPD) is the severe end of the pneumococcal disease spectrum and includes cases in which S. pneumoniae has been isolated from a usually sterile site (blood, pleural fluid or cerebrospinal ﬂuid). Clinically, these are cases of meningitis and bacteraemic pneumonia, especially in the very young, and S. pneumoniae is often the cause of bacteraemia with no obvious primary site of infection.
Local mucosal or non-invasive infection is common, such as otitis media, especially in children, and sinusitis and pneumonia (without bacteraemia) in all age groups. Rarely, S. pneumoniae may cause invasive disease such as endocarditis and deep infection in sites such as joints, the peritoneal cavity or the fallopian tubes. The incubation period of S. pneumoniae infection is variable but may be as short as one to three days.
Along with the very old and very young, patients with underlying conditions have the highest rates of disease.