Abstract: Abstract Objective: to analyse virus, mycoplasma, chlamydia and legionella infection status of community acquired pneumonia (CAP) in hospitalized patients and to provide reference resources for empirical treatment of CAP. Methods: 1079 with CAP who were treated in the respiratory medicine department of Hebei unitedu niversity affiliated hospital during October 2001 to June 2004 and January 2008 to December 2010 were objects of study. All patients’ serums were collected for IgM antibody detection (including chlamydia, mycoplasma, Legionella and virus such as EB virus, adenovirus, respiratory syncytial virus, cytomegalovirus, herpes simplex virus, Coxsackie virus, rubella virus) in the day be hospitalized and 7-10 days later at the same time data in different period were analysed. Results：In 1079 patients, positive results include 165(15.29%) chlamydia, 99 (9.18%) mycoplasma, 58(5.38%) legionella and 358(33.18%) virus. In comparison, the detection rates of legionella, syncytial virus, cytomegalovirus, herpes simplex virus, rubella virus and EB virus were decreased obviously. Antibodie’s detection rate of Coxsackie, EB and syncytial virus in the onset of 10~20 days was higher than that in 9 days ago and 20 days after. Conclusion：Atypical pathogens are important pathogenic in CAP, mycoplasma infection is one of the most common reasons. In comparison, infection rate of atypical pathogens in patients with CAP was not significantly increased and in part has a downward trend. In the onset of 20 days, most positive result of IgM antibody can be found in patient with.atypical pathogens infection.

Key words: CAP, pathogeny, chlamydia, mycoplasma, legionella, virus