A most common cause of Bronchiolitis is?

Bronchiolitis is one of the common serious acute lower respiratory tract infection in infants. Affected infants are between ages of 1 and 6 months but the disease can affect children up to 2 years old.The disease usually occurs in winter and spring. Respiratory syncytial virus is implicated in most cases. other causative agents include parainfluenza virus, influenza virus,adenovirus, Mycoplasma. protection against RSV is mediated by antibodies of IgG3 subclass.