92 INFECTION AND IMMUNITY. 



and organ of the animal is teeming with microorganisms, 

 and in so-called septicaemic diseases where the microorgan- 

 isms may be found in greater or lesser number in the blood 

 and organs. 



II. The bacteria secrete or contain in their cell-bodies 

 poisonous substances (toxins) which act deleteriously on the 

 animal economy through its own molecules. This, the chemi- 

 cal theory, is the accepted one of to-day, and finds its ready 

 explanation in nearly all infectious diseases, especially in 

 those which, like diphtheria and tetanus, may be superin- 

 duced by inoculations of cultures from which the bacteria 

 have been eliminated by filtration. The so-called toxaemic 

 diseases are so produced. 



The Avenues and Factors of Infection. 



A. Infection of the animal body is effected by one of three 

 ways : 



I. Through the respiratory tract. 



II. Through the digestive tract. 



III. Through the wounded or unwounded surface of the 

 skin or mucous membrane. 



B. Conditions and Factors. These are various and play an 

 important part in infection. Some of them have reference 

 (1) to the infecting material, or chiefly (2) to the animal ex- 

 perimented upon. To the first class belong the species of 

 bacteria, the quantity of infected material introduced, the 

 cultural conditions of the bacteria, the presence or absence 

 of the so-called mixed infection in which more than one 

 species is taking part, the method of its introduction, and, 

 in some cases, the time elapsed since the infection occurred. 

 The conditions which depend upon the animal are the follow- 

 ing : the amount of natural resistance to the bacterial poison, 

 the condition of health of the animal. 



It must be remembered that some species of bacteria are 

 much more injurious than others either on account of the 

 rapidity with which they are able to develop in the human 

 or animal economy, or on account of the large quantity of 



