Background Viral pathogens were more commonly reported than previously estimated in community-acquired pneumonia (CAP) patients. influenza like illness patients and none of 75 volunteer controls. Forty-seven CAP patients were infected by a single virus (24 influenza A virus 5 influenza B 10 parainfluenza virus type 3 [PIV-3] 2 PIV-1 2 adenovirus 2 human rhinovirus and 2 coronavirus OC43) five cases by two or three viruses co-infection. Fever?≥?39°C (66.7%) fatigue (64.6%) and purulent sputum (52.1%) was the most common symptoms in viral pneumonia patients. On multivariate analysis myalgia was included in the model for pneumonia associated with influenza infection. In the CURB-65 model only influenza infection was found independently associated with severe disease (CURB-65 score?≥?3) out of variables including age(years) sex current smoking status sick contact VER-49009 with febrile patients numbers of comorbidity presence of influenza infection presence of PIV infection with P?=?0.021 OR 7.86 (95% CI 1.37-45.04). Conclusion Respiratory virus was not a bystander but pathogenic in pneumonia and was a common cause of CAP. Keywords: Cell culture Clinical feature Community-acquired pneumonia Seroconversion Viral disease Background In China pneumonia ranks fifth among all causes of death in humans. However there VER-49009 are limited data regarding the etiology of community-acquired pneumonia (CAP) worldwide and in China with about 17% to 48% unknown . This may lead to inappropriate antimicrobial therapy and emergence of drug-resistant bacteria. Since influenza virus was first isolated in ferrets from pneumonia patients in 1933 by Smith  viral etiology of pneumonia has attracted more and more attention. Recently our ability to detect viral pathogens has dramatically improved after VER-49009 the introduction of highly sensitive nucleic amplification tests (NATs). Additionally NATs has its superiority in detection of viruses that are difficult to grow in cell culture such as human rhinovirus (HRV) human coronaviruses (HCoV) and new emerging pathogens human metapneumovirus (hMPV) and human bocavirus (HBoV). Recently epidemiological surveys on etiology of CAP showed that respiratory viruses accounted for 15% to 56% of cases [3-5]. However the real role of virus in pneumonia was few studied and still controversial [3 6 It may partially due to poor sensitivity of most viral testing assays (except NATs). However it was difficult to confirm the pathogenicity of virus tested by NATs. Thus clinical features of specific viral pneumonia were not well described [4 5 7 After combined the improvement in sensitivity and specificity of viral testing assay with more comprehensive design study more valuable information will be available. Moreover because there is limited information concerning to the prevalence and clinical features of viral pneumonia guideline of diagnosis and treatment of CAP does not provide much recommendation about the assessment and management of viral CAP. In order to better understand the real role of respiratory virus in pneumonia and better manage the patients we conducted a prospective observational study to reveal the viral etiology of adult CAP in Guangzhou as compared with etiology of patients diagnosed with influenza like illness (ILI) and with volunteer controls. Methods Patients Between April and December 2009 consecutive VER-49009 adult patients admitted to the First Affiliated Hospital of Guangzhou Medical University and diagnosed with CAP within 14?days from onset were studied. They were sampled for throat swabs at enrollment and paired sera by at least two weeks interval. CAP was defined as the presence of a new infiltrate on the BMP1 chest radiographs together with a new cough or sputum or change in respiratory symptoms or fever or sign of consolidation of lung or rales or leukocytosis (>10?×?109/L) or leucopenia (<4?×?109/L) . No alternative diagnosis was responsible to the new infiltrate during follow-up. Exclusion criteria was: 1) immunosuppression (e.g. human immunodeficiency virus infection); 2) previous organ transplantation; 3) immunosuppressive therapy.