Unprepared mice given intranasal inoculations of certain strains of pneumococci develop pneumonia. The proportion of inoculated mice which will show the pneumonia at autopsy is dependent upon the strain and type of organism and the breed of mice used. It has been shown that, with the technique employed, the pneumococci reach the lower respiratory tract and alveoli almost immediately; moreover, that an invasion of the blood stream occurs very rapidly and can be demonstrated in a third of mice during the first 10 minutes. There is some evidence that invasion of the tissues and the blood stream may occur both through the upper respiratory tract, probably the nasal mucosa, and through the alveolar walls.
It is uncertain which route of invasion, if either, is of the most importance. It has been possible to produce pneumonia by direct intravenous inoculation of pneumococci. It may be that the pneumonia is favored by a reaction at the point of invasion through the alveolar walls in the intranasally inoculated mice, but the results of the intravenous inoculation make it clear that such a local lesion is unnecessary for the production of pneumonia in mice.