Antibiotic use during the first year of life was categorized by the number of oral antibiotic prescriptions (zero, one to two, three to four, and five or more courses, as classified in other publications25 testing the association between antibiotic use and asthma development). Penicillin, cloxacillin, cephalexin, cefadroxil, and erythromycin were defined as narrow-spectrum (NS) antibiotics. The remaining antibiotics fell into the category of broad-spectrum (BS) antibiotics. Physician visits for childhood infections were classified by the number of lower respiratory tract infections (eg, bronchitis, bronchiolitis, and pneumonia), upper respiratory tract infections (eg, otitis media, pharyngitis, and sinusitis), and non-respiratory tract infections (eg, genitourinary infections, cellulitis, and impetigo). Risk and protective factors for asthma, which were derived from health-care administrative records, included gender, urban location (municipality population, > 40,000) or rural location, neighborhood income, total number of siblings at age 7 years, the number of health-care visits made during the first year of life, and maternal history of asthma (at least one physician visit or hospitalization for asthma or one prescription for an asthma drug).