Extensive bacteriologic studies of pneumonias among adults have established certain cardinal facts.
Lobar pneumonia is caused by the pneumococcus—either by one of three fixed biologic types, designated arbitrarily as Types I, II and III, or by a member of the heterogeneous group, called Group IV, which embraces all other strains.1 Under Type II a series of fixed subtypes has been shown, all closely related to the master strain; these are called atypical Type II.2
Under nonepidemic conditions the usual proportionate incidence of infections caused by the different types is about as follows: Type I, 33 per cent.; Type II, 25 per cent.; Type II, atypical, 7 per cent.; Type III, 12 per cent., and Group IV, 22 per cent.3
The average mortality of lobar pneumonia caused by pneumococcus Type I is from 25 to 30 per cent. (in cases not treated with serum); by Type II,