Virus culture (influenza A and B , adeno-, RS-, parainfluenza type 1, 2 and 3 virus) was done by using the Ohio strain of HeLa cells and human foreskin fibroplasts according to routine procedure. Viral antigens (influenza A and B , adeno-, RS-, parainfluenza type 1, 2 and 3 virus) were detected by time-resolved fluoroimmunoassay with monoclonal antibodies (17). Reverse transcription-PCR assays were used for detection of corona-, rhino-, entero-, RS-, influenza A and B as well as human metapneumovirus (hMPV). The details of methods have been discribed earlier (18, 19). Virus-specific serum antibody titers (Influenza A and B virus, adenovirus, parainfluenza type 1/3 and 2 virus, RSV, HHV-6 and enterovirus IgG as well as for enterovirus also IgM) were determined by enzyme immunoassay (EIA) using antigen-coated solid phase and horsedish peroxidase conjugated rabbit antihuman IgG (Dako, Glostrup, Denmark). Three fold increase in IgG antibody titers was considered positive except four fold increase in RSV IgG antibody titers. Enterovirus was also considered positive if virus specific IgM was positive.