MECHANISM OF INFECTION AND IMMUNITY 229 



there is no recognizable disturbance in the health of the individual, 

 either subjectively or objectively. He considers himself well and 

 attends to his usual duties, and yet this is an important and critical 

 time in the development of the infection. The bacilli are growing and 

 multiplying enormously in the man's body. They are converting body 

 proteins into bacterial proteins, native into foreign proteins, and this 

 goes on without the host being conscious of it. The ferments of the 

 bacterial cells are fitting the body proteins into the cellular molecules 

 of the bacteria. During the period of incubation, the bacterial cells 

 supply the enzymes, the body proteins constitute the substrate, the 

 process is synthetic and constructive, no poison is set free and conse- 

 quently no symptoms are manifest. It follows that the multiplication 

 of the typhoid bacillus in man's body is not the direct cause of the 

 symptoms of the disease. There is no evidence that the growth and 

 multiplication of the bacilli proceed at the expense of, or directly 

 cause injury to, body cells. The bacilli feed on the simple, soluble 

 proteins of the body. A tubercle bacillus passes through the intestinal 

 wall and leaves no lesion. A plague bacillus may penetrate the skin 

 of an animal and make no visible alteration. The rate at which the 

 virus multiplies during the period of incubation is an important factor 

 in determining the final outcome. The more virulent the virus, the 

 more rapidly does it multiply, and this means a larger amount of 

 body protein converted into bacterial protein. The phenomena of the 

 period of incubation may be studied in a guinea-pig into the abdominal 

 cavity of which a fatal dose of a virulent culture of the colon bacillus 

 has been injected. In this experiment the incubation period is from 

 eight to twelve hours, during which time the infected animal is in its 

 behavior undistinguishable from its untreated fellows. However, if 

 a drop of the abdominal fluid be taken out from hour to hour it will 

 be seen that the bacilli are multiplying rapidly. 



The Disease. In some cases the period of incubation passes 

 abruptly, in others more gradually, into that of the active disease. 

 Symptoms, both subjective and objective, develop and indicate a more 

 or less marked departure from health. In some diseases there is a 

 chill, which may vary greatly in severity, and this is followed by 

 fever. Evidently something has happened which disturbs physiologic 



