46 CLINICAL BACTERIOLOGY. 



typhoid fever, and even tuberculosis ; only for influenza, for 

 scarlet fever, and especially for measles, must a greater 

 susceptibility be assumed in the case of man. From the 

 constant contact with infective bacteria to which man is 

 continually exposed, infections would be far more frequent 

 than they really are if the predisposition of human beings 

 to bacterial diseases were not, on the whole, but incon- 

 siderable. 



In general, the power of resistance of our tissues against 

 bacteria is so great that, for infection to take place, an 

 additional special contributing cause that diminishes this 

 power of resistance in other words, a predisposing inflti- 

 ence is necessary. Among such etiologic factors expo- 

 sure to cold, traumatism, emotional disturbances, and 

 overexertion (traumatic pneumonia, traumatic tuberculosis, 

 typhoid fever after grief and worry, etc.) have long been 

 known. Further, the bacilli may gain entrance into the 

 body in especially large numbers, or they may possess in- 

 creased virulence, as is the case in the event of direct con- 

 tagion, especially in times of epidemic, when the bacteria 

 possibly have several times completed their passage through 

 the body. 



In conclusion, it may be mentioned that the infection it- 

 self may act as a predisposing factor in the development 

 of a second subsequent infection. When one variety of 

 bacteria proliferates in a body in which another variety of 

 bacteria is already in activity, the condition is designated a 

 secondary infection. Examples are afforded by some varie- 

 ties of pneumonia in cases of typhoid fever : the typhoid 

 patient is attacked by the pneumonic infection because the 

 resistance of his tissues is impaired as a result of the influ- 

 ence of the typhoid virus. Such secondary infections play 

 a most important role in connection with the infectious dis- 

 eases of man. The oral affections, the otitis, the broncho- 

 pneumonia, the cystitis, the suppurative and even the pyemic 

 processes that so frequently complicate the primary disease 

 usually are merely secondary infections ingrafted upon the 

 original disorder. A number of these complications, natu- 

 rally, do not represent true secondary infections, but only 

 secondary localizations of the original disease-causing agent. 

 Thus, pneumonia complicating typhoid fever may be due 

 to typhoid-bacilli, and otitis complicating pneumonia may 

 be due to pneumococci, etc. 



