Jessica is a 6-week-old infant who was born prematurely and weighed 1.8 kg (4 pounds) at birth. she has progressed steadily in weight and other aspects of her development. jessica also has a 4-year-old brother, sam, who attends preschool three mornings a week. recently sam has had an upper respiratory tract infection, and now jessica has been hospitalized with respiratory symptoms that are suggestive of bronchiolitis. what is the most prevalent etiologic agent causing bronchiolitis in young infants?