68o MICROBIOLqGY OF DISEASES OF MAN AND DOMESTIC ANIMALS 



the piroplasmoses and with the tsetse flies in the transmission of 

 certain of the trypanosomiases. 



The Effect of Infectious Microorganisms on the Body 



It becomes necessary to consider briefly the effect of the various 

 infectious microorganisms and their toxic substances on the body. 



The Period of iNCUBATioN.^This period is that elapsing after 

 the entrance' of the infecting organism into the body until the sjrmp- 

 toms of the disease develop. This period is variable in most diseases 

 and depends upon the same factors which modify the results of an 

 infection, namely, the virulence of the infecting organism^ the number 

 growing in the body or its tissues, the avenue> and the resistance of the 

 individual. The period can be in a measure controlled and shortened 

 in experimental animals by inoculations into the circulatory system 

 and in other regions depending on the organisms used. In some of 

 the human diseases, particularly those of unknown cause, the period of 

 incubation is quite constant, as for example, in smallpox and measles. 



Local Reactions. — ^The local effects of the toxic substances of 

 microorganisms are usually first inflammatory and later possibly ne- 

 crotic, that is, they produce a death of the tissue involved. The inflam- 

 matory changes may be confined to those of an acute character as, for 

 example, in the various serous, hemorrhagic, suppurative and fibrinous 

 inflammations, or be chronic and proliferative in nature. There is 

 always a variation in the t3T)e of inflammation depending on the loca- 

 tion of the infection and also a variation in two different individuals of 

 the same species infected at the same point with the same agenti i In 

 some diseases such as tetanus the local point of infection may-entirely 

 heal and still the bacteria be localized at this point and disseminate 

 their toxin. In some cases of tuberculosis and glanders the bacteria 

 may become localized at the points of infection and after an acute 

 inflammatory stage the point may become the seat of a chronic process 

 and proliferative changes occur in the tissues. 



General Reactions. — Metabolism. — ^The general metabolism of 

 the body is affected by the changes produced in the amount and the 

 chemical constitution of the food substances which are taken into 

 the body. By changing in the same way the substances which natu- 

 rally are eliminated from the body and by setting up new and abnormal 



