136 INFECTION 



devised for the ameba, certain trypanosomes, spirochetes, and malarial 

 plasmodia, we will be enabled to study the products of their growth or 

 of disintegration, and the immunologic agencies concerned in infection 

 and recovery; this offers a very important and fruitful field for research. 



THE COURSE OF INFECTION 



In conclusion, we may briefly consider the results of infection or the 

 general symptoms following bacterial growth and the manner in which 

 these are produced. 



The Stages of Infection. Practically all infections pass through the 

 following stages : 



1. The period of incubation, which begins at the time of infection and 

 ends with the development of the earliest general symptoms, during 

 which time the invading bacteria are multiplying in the tissues of the 

 host. During this stage no symptoms, or only those of a purely local 

 nature, are present. This period varies considerably in different in- 

 fections, and to a lesser extent in different individuals having the same 

 infection. Some bacteria may be so virulent as to overwhelm the body- 

 cells, thus making the period of incubation very short or entirely un- 

 observable. On the other hand, as, e. g., in rabies, the period may be of 

 several weeks' and, indeed, of several months' duration. In tuberculosis 

 there is usually a primary local growth, which develops so gradually 

 and the toxins are so slowly diffused that it is difficult or, indeed, im- 

 possible, to estimate the length of the period of incubation. 



According to Vaughan, during the period of incubation the bacteria 

 or their toxins or the viruses are actively engaged in changing the natural 

 body proteins into new and specific bacterial proteins, and since this 

 stage is constructive, there are no symptoms and the host is not ill. 

 Even with the experimental administration of the most poisonous of 

 toxins a definite period of incubation is usually to be observed, which 

 cannot be reduced below a certain minimum, independent of the size 

 of the dose injected; in general, however, a large dose of bacteria or of 

 toxin is likely to be followed by a shorter period of incubation than if a 

 smaller dose were administered. 



In a given case the period of incubation may be determined by 

 several factors: 



(a) The number of bacteria gaining entrance, and especially their 

 toxicity and aggressiveness. The primary factors are the degree of 

 toxicity and the amount of toxic substances produced and absorbed. 



