DISSEMINATION OF INFECTION 15 



bacteria are spread after they escape from the body in many 

 ways, the following being the most common : 

 (a.) By direct contact. 



(b.) They are carried on the hands, shoes or clothing 

 of attendants, and on farm implements, such as shovels and 

 hoes. 



(c.) They are carried in streams receiving the excreta 

 or disintegrating bodies of the infected. 



(d.) They are scattered with the excreta of birds that 

 feed upon the dead carcasses. Other animals, such as dogs 

 and foxes, are also charged with the scattering of the virus by 

 the same method. 



(e. ) The virus is often carried from one herd to another 

 by introducing chronic cases or those already infected in which 

 the symptoms have not yet appeared. 



(f. ) Animals are often infected by shipping them in cars 

 or crates that have previously contained diseased animals and 

 that have not been thoroughly disinfected. 



(g. ) The pathogenic protozoa are transferred from 

 infected to non-infected individuals by means of insects. They 

 are carried from place to place in infected animals. 



§ 8. Cause for the variations in the course of an 

 infectious disease. It is a recognized fact that there is much 

 variation in the course of infectious diseases in different epizo- 

 otics and often marked individual variations occur in the same 

 outbreak. In explaining this interesting phenomenon, it is 

 important to take into account the question of individual 

 resistance or immunity, — partial or more complete. It was 

 found in case of certain diseases that when an individual was 

 partially immunized and then infected, that the lesions were 

 very much modified. The teachings of a specific etiology 

 point to this phenomenon as a result of certain- biological or 

 vital differences existing either in the parasite or in the host, 

 possibly in both. The results of the investigations already 

 made along this line suggest as a probable explanation, that 



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