The most common etiological agent for acute bronchiolitis in infancy is:

Acute bronchiolitis is a serious acute lower respiratory infection in infants affecting children between 1 and 6 months of age. It is most commonly caused by Respiratory syncytial virus. Other causative organisms includes Parainfluenza virus 3, 1 and 2, adenovirus, influenza virus and rarely Mycoplasma pneumonia. Ref: Essential Pediatrics, 6th Edition By O P Ghai, Pages 352-4; Nelson's Essential of Pediatrics, 17th Edition, Pages 1415-16.