The World Health Organization continues to address the high mortality rate from acute respiratory infections in developing countries. Viral and bacterial acute respiratory infections are now known to be almost equally common in those countries where studies have been undertaken. The proper identification of seriously ill infants—many of whom have acute respiratory infections—by health workers and parents is a subject of health programs worldwide. The role of respiratory infection in sudden infant death is still unclear. Infants at risk for serious acute respiratory infections are those who are malnourished, have chronic chest or heart disease, and are immune deficient. These groups would benefit most from the development of vaccines for respiratory viruses and from ribavirin therapy.