Acute bronchiolitis in infants, mainly caused by the Respiratory Syncytial Virus (RSV), is a public health issue, in terms of morbidity and hospital costs. Nirsevimab, a long-acting antibody against RSV available for all infants under 12 months, could profoundly modify the epidemiology of the next epidemic seasons, with the increase of the frequency of the other respiratory viruses (endemic Coronaviruses, Metapneumovirus, etc) Monitoring viral ecology is important, as the impact of respiratory infections on morbidity in the short, medium and long term, but also in economic terms. Infants under the age 12 months with a first episode of bronchiolitis and consulting the emergency pediatric department will be included. A nasopharyngeal swab will be performed as routine care, and clinical data will be collected. Multiplex Polymerase Chain Reaction will be performed to identify respiratory virus(es) responsible for the bronchiolitis. In hospitalized infants with RSV or rhinovirus positive Polymerase Chain Reaction (PCR), daily nasal washes will be collected to perform viral loads with specific quantitative PCR (nasal washes biobank), associated with daily clinical data. A 12-month follow-up is planned for every infant included, consisting with 4 phone calls, to identify factors associated with the onset of preschool wheezing. The main objective is to describe the viral epidemiology associated with acute bronchiolitis leading to a pediatric emergency department visit the implementation of Nirsevimab.