272 PARASITIC BACTERIA 
the whole body, located at no particular spot (blood poisoning), 
while in other cases they may be definitely localized at some one 
place (diphtheria). Between such extremes there are many inter- 
mediate types. Whenever the microorganisms multiply in the 
body they produce chemical changes, just as they do elsewhere. 
New chemical 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 itself, this poisoning is easy to understand, but localized 
diseases are similarly explained. Diphtheria, for example, is pro- 
duced 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 ab- 
sorbed 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 character- 
istic of the various diseases. 
RESISTANCE AGAINST MICROORGANISMS 
A very large majority of microdérganisms 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 search- 
ing, 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 
