Infection with RSV is the most common cause of respiratory tract illnesses (LRIs) in the first 3 years of life. There are significant social and health care costs associated with RSV-LRIs. More than 3% of US children are hospitalized each year due to RSV and 500 die annually. Several longitudinal studies have also suggested that children who have RSV-LRIs are at substantially increased risk of developing asthma in the first 3 years after infection and bronchial hyperresponsiveness (BHR) many years after the primary infection. Mechanisms involved in RSV disease are not well understood. Recent reports suggest that RSV may initiate the innate immune response through the pattern recognition receptor, Toll like receptor-4 (TLR4). In this project we will study the capacity for single nucleotide polymorphisms (SNP) in TLR4 gene to induce varying levels of inflammatory chemokine and cytokine production. It has been suggested that such a mechanism may result in altered immune responses to RSV infection and different clinical outcomes. This research has direct application to improving our understanding of bronchiolitis in early childhood, particularly those factors that influence severity of the disease, and may have implications for possible therapy of patients with bronchiolitis in the future.