228 MECHANISM OF INFECTION AND IMMUNITY 



So great is the bacteriolytic action of the blood that even some patho- 

 genic bacteria do not infect when injected directly and wholly into 

 the blood. This is true of the bacillus of symptomatic anthrax. A 

 dose which infects when administered subcutaneously, fails when given 

 intravenously. The cholera bacillus is harmless when introduced sub- 

 cutaneously in doses which would infect by the intestine. In the first 

 instance, it is speedily killed by the bactericidal constituents of the 

 tissues ; in the second it grows and multiplies in the intestine in which 

 it does not come in contact with the germicidal agents. 



There are many conditions which influence the capability of bac- 

 terial growth in the animal body. A given bacterium may be patho- 

 genic to one species of animal and without effect on another. Some 

 are active in mixed cultures, one bacterium being of assistance to 

 another. Some grow in certain tissues of the body and not in others. 

 The number of bacteria introduced into the animal is an important 

 factor. One anthrax bacillus may kill a mouse and one tubercle 

 bacillus may have a like effect on a guinea-pig, but these are exceptions, 

 and whether or not an infection results depends, in most instances, in 

 part, on the number and virulence of the organisms introduced. 



While the blood has a marked bactericidal action on some bacteria, 

 it forms an excellent culture-medium for others. Virulent streptococci, 

 plague and tubercle bacilli grow abundantly in the blood and kill the 

 more promptly the sooner they find their way into the circulation. Quite 

 naturally, many bacteria grow most vigorously in injured and necrotic 

 tissue on account of the lessened resistance. The readiness with which 

 streptococci take possession of tissues already weakened by cancer, 

 tuberculosis or syphilis is an illustration. 



Incubation. The period of incubation of an infectious disease is 

 the time interval between the introduction of the infecting agent 

 and the first appearance of the symptoms of the disease. This varies 

 greatly in different diseases and for the same diseases in different 

 animals. With the same disease in the same species there are also 

 variations, but not so marked. For instance, one swallows typhoid 

 bacilli, he does not develop fever the same day or the next, but as a 

 rule between the sixth and tenth day. In some individuals the period 

 of incubation for this disease may be longer. During this period 



