Enrolment Infant Urine is collected from hospitalized infants during the acute infant illness at study enrolment, and from a convenience sample of outpatient subjects. Urinary measurements including, leukotrienes C4/D4/E4 (LTC4/D4/E4), and urinary metabolite of the isoprostane, 15-F2t-IsoP (8-iso-PGF2a) will be measured by a gas chromatographic, and other biomarkers and the remainder of the urinary biospecimens will be maintained in the repository. 25, 26 IgE, immunoglobulin E; LRTI, lower respiratory tract infections; URI, upper respiratory tract infection.