The primary objective of the study is to decrease the proportion of infants with acute bronchitis during the first year of life by supplementation of 1,000 IU/day vitamin D. Secondary otcomes are: To check that the administration of 1,000 IU/day vitamin D decreases the proportion of infants with upper respiratory tract infections, the proportion of children under one year of age hospitalized for acute bronchiolitis, and the demand on the healthcare system due to respiratory infections and absences from work for parents and achieves a higher proportion of children with adequate blood levels 25 OH vitamin D.