AZ-SWED is a parallel group, double blind, placebo control efficacy clinical trial with two separate hypotheses. The trial will compare the 5-day outcome of preschool children presenting to an Emergency Department (ED) with an acute, severe wheezing episode and treated with either once daily oral Azithromycin (12 mg/kg/day for 5 days) or placebo. The AZ-SWED researchers will make separate comparisons in children in whom specific pathogenic bacteria are isolated from nasopharyngeal swabs, and in those in whom they are not isolated. The primary outcome will be the Asthma Flare-up Diary for Young Children (ADYC), a validated instrument that caregivers will transmit electronically daily after discharge from the ED. Families will be contacted daily during the five-day treatment to collect the ADYC, and to assess compliance and complications. A randomly chosen subset of enrolled children will participate in two follow-up visits 5-8 days and 14-21 days after visit 1 to assess development of resistance to study drug and treatment response related changes in the airway microbiome.