 There is a growing body of data suggesting that vitamin D modulates the hosts immune response to acute respiratory infection ARI. The primary aim of this study is to determine whether maternal vitamin D3 supplementation versus placebo decreases the incidence rate of microbiologically confirmed viral-associated ARI among infants in Dhaka, Bangladesh. Secondary outcomes include: A incidence of ARI associated with specific major pathogens, B incidence of clinical ARI without the need for positive microbiology, and C quantitative density of pneumococcal carriage. Infants will be followed from birth until 6 months of life. Among infants who meet at least one of the specific case definitions for ARI see Detailed Description section, nasal swab specimens will be collected. Respiratory samples will be analyzed by real-time polymerase chain reaction qPCR to identify a 7-virus panel influenza A and B, respiratory syncytial virus, human metapneumovirus, adenovirus, and parainfluenza types 1, 2, and 3 plus quantitative density of S. pneumonia. 