Bronchiolitis is one of the most common acute illnesses in infancy and the leading cause of hospitalization in this age group.    The annual hospitalization rate for bronchiolitis is estimated to be 30 to 40 per 1000 infants. The annual rate of hospitalization for respiratory syncytial virus (RSV) bronchiolitis is estimated by ICD-9-CM codes at 6.7 per 1000 children under 5 years and 26 per 1000 infants.  The hospitalization rates for confirmed RSV infection obtained by prospective, population-based surveillance are 3 and 17 per 1000 children under 5 years and under 6 months, respectively.  The rates for hospitalization due to RSV bronchiolitis have remained relatively stable in the United States through the 1997 to 2006 time period.  It is estimated that RSV infection alone is responsible for approximately 1 of 334 hospitalizations, 1 of 38 visits to an emergency room, and 1 of 13 visits to a primary care practice each year in the US.  The incidence of bronchiolitis displays a distinct seasonal pattern, with most cases in the US occurring from November to May.  The peak incidence of the disease usually occurs in January or February. In the southeast, the onset and peak of infections is slightly earlier. The incidence in other parts of the world follows a similar pattern. The incidence of RSV infection is remarkably similar year to year. In contrast, parainfluenza-1 infections (causing croup) display a biennial incidence pattern.
Bronchiolitis is almost exclusively an infantile disease, and by 3 years of age essentially all children have serologic evidence of having been infected with RSV. However, primary infection with RSV in infants does not confer protective immunity, so repeat infections are common. Although in most infants the disease is mild and self-limited, severe disease can occur, especially in infants under 6 months of age.  Infants with underlying risk factors for severe infection, such as prematurity, congenital heart disease, or chronic lung disease, have a greater risk of hospitalization, but the majority of hospitalizations are in infants with no underlying risk factors. In addition to acute effects of bronchiolitis, a large proportion of infants who have bronchiolitis related to RSV infection go on to have recurrent or chronic wheezing for many years. Some studies suggest that their risk of allergy and asthma is also increased.