Acute bronchiolitis is one of the common acute respiratory infections in children less than 2 years of age.It is a major cause of morbidity and mortality among children of this age group. It is an inflammatory injury occuring to the bronchioles ususally caused by a virus infection usually Respiratory syncitial virus. Though this condition may occur in any age group it is more severe in younger infants as the older airways of older children can better accomadate the mucosal edema. Bronchiolitis usually affects infants lesser than 2 years of age, peaking between 3 - 6 months. It is usually a self- limited condition caused by RSV.The aim in the treatment of bronchiolitis will be more standardized care, fewer hospitalizations, better management of resources, and shorter length of hospital stays without increasing readmission rates or decreasing family satisfaction. Bronchioles are small airways less than 2mm in diameter and do not have cartilage or submucosal glands.The effects of bronchiolar injury may begin 18 to 24 hours after the infection.Increased mucus secretion,Atelectasis, Air trapping,Bronchial obstruction and constriction, Reduced ventilation that leads to ventilation-perfusion mismatch can be seen 18-24 hours after the infection. Type 1 allergic reactions involving IGE are implicated in bronchiolitis. Babies who are breast fed receiving colustrum and IgA are protected from broncholitis. A necrosis of respiratory epithelium occurs. Also there if proliferation of goblet cells with increased mucus production.Epithelial regeration with non ciliated cells results in impaired elimination of secretions. Lymphocytic infiltration results in mucosal edema. Small bronchiole epithelium was circumferentially infected but basal cells were spared. Both type 1 and type 2 alveolar pneumocytes were also infected.airway obstruction was due to epithelial and inflammatory cell debris mixed with fibrin, mucus, and edema fluid but not to bronchial smooth muscle constriction. neutrophil inflammation, but not eosinophil inflammation, is related to the severity of a first infection in infants.