INFECTION 241 



enters in a variety of ways, as through wounds, by inhala- 

 tion, or by invest ion. The tetanus or lockjaw bacillus is 

 always introduced through wounds of the skin or of the 

 mucous membrane. 



The original or initial infection is called the primary in- 

 fection, and may be 1'ollowed by a second invasion with 

 another kind of organism which may have been present in 

 the body, but which was unable to multiply until the re- 

 sistance was first lowered by the primary infection. Infec- 

 tion is usually due to a single specific organism, but some 

 troubles are due to a mixed infection with two or more or- 

 ganisms. When the body has been weakened by organic 

 diseases, it is sometimes more susceptible to invasion by 

 certain disease-producing organisms, or the results of such 

 invasion an- more likely to be serious. The original trouble 

 might have resulted in death, but the end is hastened by 

 the terminal infection, as it is called. 



The invading organisms injure the tissues by the pro- 

 duction of poisonous substances known as toxins. In some 

 cases the organisms grow in a limited area, and do not cause 

 any great destruction of the tissue at the point of growth ; 

 but the toxin is so active that a minute quantity is suffi- 

 cient to cause death. Such a disease is known as toxemia. 

 in contradistinction to the bacteremia or septiccemia, in 

 which the entire body is invaded by the organism, as in the 

 case of anthrax. Examples of toxemias are lockjaw and 

 diphtheria. In still other instances the invasive powers of 

 the organisms are not great, but the tissue is destroyed at 

 the point of growth, as in the case of the pus-producing 

 organisms. 



It is evident that the symptoms of any disease can not 

 appear until the organism has had time to multiply and to 

 form sufficient toxin to have a visible effect on the body 

 of the animal. This period which may vary from a few 



