To investigate the epidemiological association of respiratory infection with viral load, HBoV-positive patients were categorized into low-and high-viral-load groups by using 1.0 × 10 6 copies/mL as a threshold value. The detection rate of HBoV infection was at its peak in the first year of life (rate of detection, 8.6% between the ages of 6 and 12 months), as in the cases of RSV or PIV infection. Most of the HBoVpositive patients aged less than 3 yr belonged to the high-viral-load group. The detection rate was lower (1.5%) in patients aged more than 10 yr, and these patients belonged exclusively to the low-viral-load group. HBoV is rarely de-tected in adults except in cases of immunosuppression [10, 16, 17] . The lower detection rate and viral load of HBoV in older patients may be attributed to immunity acquired from an infection at a younger age. A seroepidemiologic study of HBoV showed that 5.6-83.3% of children aged 6 months-3 yr were seropositive for HBoV [18] . Lau et al. [19] suggested that HBoV infection might develop only once because of the subsequent development of life-long immunity conferred by neutralizing antibodies produced in response to the infection.