INFECTION. 39 



which is usually induced through the action of cold. The 

 disease may be transmitted also from one pneumonic pa- 

 tient directly through the sputum to other individuals. 

 Instances of house-epidemics of pneumonia have been re- 

 ported, and which are capable of scarcely any other expla- 

 nation than that the bacteria were present normally in the 

 upper air-passages of the individual first attacked, and 

 whence they have gained entrance into the pulmonary tis- 

 sue, whose resistance has been diminished in consequence 

 of the action of cold, and here have set up an inflammatory 

 process. The bacteria, however, that are obtained from a 

 focus of disease are more virulent than those that are found 

 upon normal skin and mucous membrane. The virulence 

 of the causative bacteria, which is responsible for the in- 

 tensity of the disease, is increased in turn with the severity 

 of the disease -process. This is true during the height of 

 the infection for all bacteria. With the decline of the dis- 

 ease, naturally, when the bacteria have remained for a cer- 

 tain time in the organism now acquiring immunity, their 

 virulence lessens, as a rule. Returning to our illustration 

 of pneumonia, the course of events would be that the 

 pneumococci derived from the pneumonic lung of the indi- 

 vidual first affected would now be capable, by reason of 

 their increased virulence, of infecting a second and a third 

 individual without the aid of predisposing influences. 



A striking illustration of the variation in virulence of mi- 

 croorganisms within the living body is furnished also by the 

 pyogenic streptococci and staphylococci. These are found 

 in association with all possible inflammatory and suppura- 

 tive processes, from a simple panaris to the most intense 

 septicemia or pyemia. Their morphologic peculiarities re- 

 main the same throughout. A difference is apparent only 

 in animal experimentation. The cocci obtained from the 

 benign affections prove slightly, if at all, virulent ; while 

 those obtained from severe infections induce the most dele- 

 terious consequences in experimental animals. 



(b) The Amount and the Purity of the Infectious 

 Material. To induce infection experimentally in animals, 

 a definite amount of the culture is always necessary, and 

 this varies for different bacteria and individual species of 

 animals, although it is almost constant for the same variety 

 of animals and of bacteria. If a smaller amount of bacteria 

 is used, infection will not take place. This relation is 



