Bronchiolitis is an acute viral disease of lower respitatory pathways in children under 2 years of age. It is manifested by coughing, whistling and refusing food intake. The most common cause of bronchiolitis is a respiratory syncytial virus. Vitamin D is a steroid hormone which is synthesized in the skin after sun exposure or food intake. With discovery of Vitamin D receptors and hydroxylated enzymes in immune cells, a potential role for Vitamin D in maintaining immune homeostasis was seen. It has been established that Vitamin D induces a change of the antimicrobial polypeptide, cathelicidin, which has both antibacterial and antiviral roles.