Respiratory syncytial virus (RSV) is the most common pathogen causing disease in children worldwide. In the US, RSV infection is the leading cause of infant hospitalization. A small group of diagnostic codes--RSV alone, bronchiolitis secondary to RSV, unspecified bronchiolitis and unspecified pneumonia--are found in at least 120,000 annual admissions in the US, 25% of all of pediatric hospitalizations. The cost of treating RSV infection has been estimated at $1 billion in 2002. There are annual winter epidemics of RSV in North America, but the start time of the epidemics, their duration, and severity (measured by number of hospitalizations) vary from area to area and year to ...