Mycoplasma is a small, free-living, motile bacterium that lacks a cell wall. Mycoplasma pneumoniae is found worldwide and may cause up to 20% of pneumonias. Infection is endemic but epidemics also occur, the infection being spread by airborne droplets between close contacts. M. pneumoniae infection is found most commonly in childhood, adolescence and early adulthood. The oropharynx, trachea and bronchi are commonly involved, but there is frequently infiltration into the lung, causing pneumonia. The clinical features and treatment are described on M. hominis and Ureaplasma urealyticum cause nonspecific urethritis and cervicitis.