To investigate the clinical, etiological and epidemiological characteristics of acute lower respiratory tract infection (ALRI) in Kunming region. The etiological agents, including bacteria, virus and atypical pathogens were studied in order to determine different characteristics of age distribution of respiratory infectious agents.

From October 2005 to October 2007, a total of 1402 hospitalized children with ALRI were included into this study (study group). Meanwhile, another 50 healthy children who underwent regular physical examination were recruited into control group. The study protocol was approved by the Ethical Review Board of Investigation in Human Being of Children's Hospital of Kunming. Informed consent was obtained from each participate. There had no significant difference between two groups on ages and so on. The nasal secretion samples were collected from study group for the bacterial agents culture and identification; viral antigens were detected by direct and indirect immunofluorescent analysis.

The etiological agents were detected in 402 of 1402 patients (28.79%), of which 9.1%(127/1402) were mixed infections. The bacterial-, viral-, and non-typical etiological agents were identified in 402 (28.79%), 365 (26.0%) and 259 (18.5%) of 1402 patients with ALRI, respectively. Total positive rate of respiratory pathogens was 82.2% (1153/1402). The bacterial agents identified from the 402 positive samples mainly consisted of the Gram-negative(G^-) bacteria(73.1%) [E.coli (41.3%), Klebsiella pneumoniae (15.4%), Haemophilus inflenzae (6.0%)], and the G⁺ bacteria (mainly Streptococcus pneumoniae) was detected as causative agents responsible for ALRI, for incidences of 14.4%. Of 365 virus positive samples, 312 samples (85.5%) were tested as positive for the respiratory syncytial virus (RSV) antigen, 12 samples testing positive for influenza A virus(IVA), 7 samples for parainfluenza virus type (PIV)Ⅱ, 24 samples for PIVⅢ and 7 samples for adenovirus (ADV). No samples testing positive for influenza B virus (IVB). Non-typical pathogens identified were Mycoplasma pneumoniae(MP), Legionella pneumoniae type 1(LP1), and Chlamydia pneumoniae(CPn), accounting for 11.6%, 6.3%, and 0.6% of the total samples, respectively. Bacteria, virus were two main causative agents responsible for ALRI in younger children, whereas non-typical pathogens were frequently observed in elderly children. Bacteria and atypical pathogens infection incidences showed seasonal differences during a year, but the RSV was the main causative agent during a year, especially in autumn.

G^- bacteria and respiratory syncytial virus are the two main local causative agents which were responsible for ALRI in children; the younger the children are, the higher incidence there is. The etiological agents' patterns vary from place to place.