The most common etiological agent for acute bronchiolitis in infancy is –
"The most common etiological agent for Bronchiolitis is Respiratory syncytial virus. RSV is responsible for more than 50% of cases of bronchiolitis."

Bronchiolitis 

Bronchiolitis is the most common serious acute lower respiratory tract infection in infants and young children.
The most vulnerable group is between the ages of 1 and 6 months.
But the disease can affect children up to 2 years.
More common in males.

Organisms causing bronchiolitis

Bronchiolitis is predominantly a viral disease.

i) RSV (mostcommon) 
ii) Parainfluenza virus 3, 1, 2       
iii) Adenovirus    
iv) Influenza virus
v) Mycoplasma pneumoniae
Influenza virus
Para influenza virus
Rhinovirus
Respiratory syncytial virus