252 PARASITIC BACTERIA. 



(diphtheria). Between such extremes there are many intermediate 

 types. Whenever the microorganisms multiply in the body they 

 produce chemical changes, just as they do elsewhere. New chemi- 

 cal bodies are secreted by them and among these, in the case of 

 disease germs, there are some that are poisonous in their nature. 

 Such substances are called toxins. Wherever they are produced 

 they are liable to be absorbed by the blood, and the body may thus 

 be directly poisoned by them. If the bacteria are in the blood it- 

 self, this poisoning is easy to understand, but localized diseases are 

 similarly explained. Diphtheria, for example, is produced by 

 bacteria growing on the inside surface in the throat. The bacteria 

 themselves do not enter the body, but, growing in the throat, they 

 develop very powerful toxins, and these are absorbed into the blood, 

 producing a general poisoning of the whole body. All disease 

 germs produce poisonous materials which are absorbed by the 

 body, and these cause the direct injury characteristic of the various 

 diseases. 



RESISTANCE AGAINST MICROORGANISMS. 



A very large majority of microorganisms are quite unable to live 

 within the bodies of living animals or plants, and therefore are not 

 parasitic. If common putrefactive bacteria be inoculated into the 

 blood of a living cow or into her flesh, they will speedily die without 

 multiplying, disappearing in a very short time. If these same 

 bacteria are inoculated into the same animal after it is dead, they 

 will grow with rapidity, quickly causing the flesh to putrefy. Why 

 is there this difference ? The complete answer to this question is one 

 for which bacteriologists have long been searching, but have as 

 yet only partly found. A partial answer is that the living tissues 

 contain substances that are injurious to the bacteria. What these 

 substances are, how they act, why they disappear after death, and 

 numerous other questions concerning them are among the most 

 important of the problems before bacteriologists to-day. With these 

 complicated questions, however, we are not concerned in this work. 



It is evident enough that some kinds of microorganisms can 



