538 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



In conclusion, should be mentioned one other indirect way in which 

 infectious agents are eliminated from the body, namely, by being taken 

 up by suctorial insects from the blood. It is necessary that this be done 

 in order to perpetuate the parasite and complete its life cycle in certain 

 instances, as with the mosquitoes in yellow fever and malaria. In others 

 the parasites are only taken up by the insect and subsequently injected 

 into another individual. 



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 microorganism into the body until the symptoms 

 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, virulence, number, avenue and 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 microorganism. 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 micro- 

 organisms are usually first inflammatory and later possibly necrotic. 

 The inflammatory changes may be confined to those of an acute character 

 as, for example, in the various serous hemorrhagic, suppurative and fibrin- 

 ous inflammations or chronic and proliferative in nature. There may 

 be a variation in the type of inflammation depending on the location 

 and also a variation in two different individuals of the same species, 

 infected at the same point with the same agent. 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 

 set in. 



GENERAL REACTIONS. Metabolism. The general metabolism of the 

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



