M. pneumoniae infection is distributed worldwide. The organ-ism is responsible for up to 20% of community-acquired pneu-monia, which requires hospitalization. M. pneumoniaeinfection occurs both in epidemic and in endemic forms. The condition is widely documented in the European countries and in the United States. The exact information on M. pneumoniae infec-tion is not available from the developing countries. However, the results of few seroprevalence studies indicate that the con-dition may be endemic in many of the developing countries.
M. pneumoniae is a strict human pathogen. The bacteria inhabitthe upper respiratory tract in an infected host. They are usu-ally present in the mucosa, residing extracellularly on the upper respiratory tract.
Humans are the usual host of M. pneumoniae and thus signifi-cant reservoir of infection. Patients with active infection are more likely to transmit M. pneumoniae infection. M. pneumoniae is most commonly transmitted by close contact through nasal secretions. The infection is transmitted by inhalation of aero-solized droplets. Person-to-person transmission usually occurs among college students and military recruits who live together in close proximity. M. pneumoniae is usually associated with pneumonia, and highest rate of infection is found in children between 9 and 10 years, and also in young adults. Infection is common among school-going children
In recent years, the infection is also being increasingly docu-mented in people older than 65 years. In this old population, M. pneumoniae is responsible for causing nearly 15% of commu-nity-acquired pneumonia and is second pathogen next only to Streptococcus pneumoniae as a cause of pneumonia.