216 BIOLOGY OF PNEUMOCOCCUS 



more conservative estimate of 20 per cent. Be that as it may, since 

 the differentiation of the species into the many serological types, 

 the list of publications which have appeared reveal the widespread 

 prevalence, wherever investigated, of pneumococci of one type or 

 another as members of the normal bacterial flora of the mouth. 

 There is some doubt, however, as to how far down in the respira- 

 tory tract of healthy individuals pneumococci may be found in a 

 state of saprophytism. Pneumococcus under favorable conditions 

 may exchange this benign vegetative existence for a life of malig- 

 nity for man. According to Hiss and Zinsser — who voiced a gen- 

 eral opinion — lobar pneumonia is the type of infection most often 

 caused by Pneumococcus. McLeod differed and expressed the be- 

 lief that "the most frequent lesion produced by the organism is a 

 catarrhal condition of the upper respiratory tract involving the 

 larger bronchi, associated with slight or considerable pyrexia, and 

 often popularly described as 'influenza.' " The truth of the mat- 

 ter is of less concern to the bacteriologist than to the clinician. On 

 the other hand, the fact is incontestable that the great majority 

 of cases of lobar pneumonia are caused by Pneumococcus. In 

 addition, Pneumococcus may be the sole causative factor in bron- 

 chopneumonia, sinusitis, otitis, mastoiditis, meningitis, peritonitis, 

 nephritis, arthritis, and infections of the eye and other tissues. 

 Some of the manifestations may be primary in character or 

 secondary to pneumococcal disease in the lung. In company with 

 organisms of other bacterial species, Pneumococcus may compli- 

 cate or intensify the severity of the infection. 



Etiology of Pneumonia 



PNEUMOCOCCAL TYPES IN LOBAR PNEUMONIA 



The predominance of Pneumococcus as the causative agent in 

 lobar pneumonia with the percentage distribution of other bac- 

 terial species is seen in the table supplied by Heffron. 601 



