This study was undertaken to determine whether there is, in infancy, an immunity to the fixed types of pneumococci. The results we believed might help to explain why bronchopneumonia is relatively frequent and lobar pneumonia relatively infrequent during the first years of life.
The possible reasons why bronchopneumonia is the most common type in infants, in debilitated children and those suffering from certain other infections, while lobar pneumonia selects as its victims older children in good physical condition, has long been of interest to us. Various theories have been advanced but none seem to have offered sufficient evidence for confirmation. The fact that pneumococci are practically always the cause of the lobar type, while other organisms either alone or associated with pneumococci have been isolated in cases of bronchopneumonia, suggested the possibility that infants may have an immunity to pneumococcic infections. The immunity of infants to certain other infections such