Respiratory syncytial virus (RSV) is an endemic virus, present throughout the world, the number one pathogen responsible for acute respiratory infections in children. In recent years something has led to a radical epidemiological change in the presentation of the virus, let's try below to understand what the real factors are that led to all this: immune debt or changed patterns of human-to-human contact? What role does the maturation of the child's airways actually play in limiting the risk of RSV infection?