EPIDEMIOLOGY OF PNEUMONIA 475 



virulent types I, II, and III, may disappear from the mouths of con- 

 valescents within three or four weeks, and sometimes sooner, and be 

 supplanted at such times by the less virulent normal type IV strains. 

 Stillman has also shown that individuals associated with pneumonia 

 patients may frequently harbor organisms of the same type as those 

 infecting the patients, and he has found organisms corresponding to 

 the patients' type in the dust of the sickroom. It seems unquestion- 

 able, therefore, that there may be carriers of virulent pneumococci 

 entirely analogous to the carrier states developed with meningococci 

 and other organisms. 



While autoinfection, therefore, cannot be completely excluded, it 

 seems probable that the origin of most cases of lobar pneumonia is best 

 explained by the acquisition of a virulent pneumococcus strain, either 

 directly from a case or from a carrier, with a depression of resistance 

 due to cold, exposure, etc., coincident with the presence of this virulent 

 strain. In the light of these facts it is clear that our sanitary point of 

 view in regard to pneumonia must be materially changed. We can 

 now understand why localized epidemics have been so often observed 

 in institutions, war hospitals, and other crowded communities, and can 

 justly evaluate the importance of the transmission factor in the spread 

 of this disease. In outlining sanitary procedures for any disease it is 

 of the utmost importance that such a thorough understanding of the 

 relative importance of transmission and the susceptibility factor 

 should be acquired. It is never possible to carry out all desirable 

 measures of prevention completely, and it is, therefore, necessary to 

 know definitely upon which factors the greatest stress must be laid in 

 planning the sanitary campaign. 



In all communicable diseases the two factors which influence spread 

 are, in the first place, the transmission of the virulent organisms, and, 

 in the second place, the susceptibility of the recipient. When trans- 

 mission becomes general and community susceptibility is normally 

 high, as in plague, typhoid, cholera, etc., epidemics are bound to spread 

 rapidly. There are diseases like those mentioned above, as well as 

 smallpox, measles, scarlet fever, and influenza, in which the suscepti- 

 bility of the normal, previously unexposed individual is so great that 

 hardly anyone sufficiently exposed, will escape. It is plain that in such 

 diseases sanitary measures must be aimed particularly at the preven- 

 tion of transmission, with, wherever possible, artificial immunization 

 of the community. There are other infections, however, chief among 

 which we believe is pneumonia, in which the resistance of normal 



