The outcome variables of interest are the incidence of asthma and allergic rhinitis. The primary exposure variables of interest are the severity and aetiology of the infant viral illness and maternal and familial atopic status and other environmental exposures. Cumulative asthma incidence over time, taking into account loss to follow up, will be used for illustrating incidence data. Incidence of asthma and allergic diseases among the enrolled infant population with viral respiratory illness will be calculated by dividing the number of incident asthma cases by the person time of follow up. A Kaplan-Meier plot of cumulative incidence over time, taking into account loss to follow up, will be used for illustrating incidence data. Incidence rates will be calculated by dividing the patient population into quartiles/quintiles of bronchiolitis severity scores. The adjusted risk of asthma and allergic rhinitis with bronchiolitis severity will be evaluated using the Cox proportional hazard regression model.