The first isolates of Chlamydiapneumoniae were obtained serendipitously during trachoma studies in the 1960s. After the recovery of a similar isolate from the respiratory tract of a college student with pneumonia in Seattle, Grayston and colleagues applied the designation TWAR after their first two isolates, TW-183 and AR-39. C. pneumoniae appears to be a common human respiratory pathogen. The mode of transmission remains uncertain but probably involves infected respiratory tract secretions. Spread of C. pneumoniae within families and enclosed populations, such as military recruits, prisons, and nursing homes, has been reported. The proportion of community-acquired pneumonia in children and adults associated with C. pneumoniae infection has ranged from 0% to >44%, varying with geographic location, the age group examined, and the diagnostic methods used. Early studies that relied on serology suggested that infection in children younger than 5 years was rare; however, subsequent studies using culture and/or polymerase chain reaction (PCR) have found the prevalence of infection in children beyond early infancy to be similar to that found in adults.

Studies that have used culture have found a poor correlation with serology, especially in children. Although 7% to 13% of children 6 months to 16 years of age enrolled in two multicenter pneumonia treatment studies were culture positive and 7% to 18% met the serologic criteria for acute infection with the microimmunofluorescence (MIF) test, they were not the same patients. Only 1% to 3% of the culture-positive children met the serologic criteria, and approximately 70% were seronegative. By age 20, approximately 50% of persons will have detectable anti-C. pneumoniae immunoglobulin G (IgG). Seroprevalence may exceed 80% in some populations.