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 
