Serum specimens were collected from 386 healthy individuals aged 0 to 70 years in 2008; 244 specimens were from infants and children who visited Beijing Children's Hospital for regular health check-ups. The 142 specimens from adults were provided by the Beijing Blood Center. Exclusion criteria for all subjects included pregnancy, any abnormalities in renal and liver function tests, HIV/AIDS, sexual transmitted diseases, tumor, recurrent or acute infection, and medication. None of the subjects had any respiratory infection for at least three months prior to the blood samples being taken. In addition, paired acute-phase (at the time of admission) and convalescent -phase (2 weeks after disease onset) serum samples were collected from 31 children (median age 17 months; range of 1 month to 9 years) with acute lower respiratory tract infections (ALRTIs) when they were hospitalized at the Beijing Children's Hospital. The DNA of HBoV1, but not HBoV2-4, was detected in the nasopharyngeal aspirates of these 31 patients at the time of admission by nested PCR and sequence analysis using primers targeting the viral proteins (VP) 1/2 region [9] . All serum samples were stored at 280uC prior to use.