PATHOGENESIS AND CULTIVATION. 143 
and the bacteria gaining access to the blood current be carried to other 
ports of the body and grow, producing new foci of necrosis. Some 
forms seem to incite an inflammatory change with the formation of 
new tissue. 
What factors influence the development of various pathogenic 
bacteria in the body? 
The number of bacteria present; the varying virulence shown by 
the same organisms at different times; the capacity of resistance of 
the body-cells. 
What is meant by natural immunity against bacterial infection ? 
Different animals are affected differently by the same organism, As 
for example, the same micro-organism that will grow and cause disease 
in one variety of animals may have no effect whatever upon individuals 
of another kind. The latter then are said to have a natural immunity 
from that particular organism, and this probably depends upon the 
peculiar composition of their tissue juices and the behavior of their 
cells. 
What is an acquired immunity ? 
There are some diseases affecting the human race of which one 
attack protects the individual from after attacks, thus producing an 
artificial immunity by natural methods. This is true of the infectious 
exanthemata, Artificially acquired immunity may be produced against 
some of the infectious bacteria by inoculations with the attenuated 
organisms, repeated again and again with stronger and more virulent 
doses each time. 
What are Koch’s rules to prove the pathogenesis of a given 
organism ? 
It must be proved to be present in all cases of the disease in ques- 
tion. It must be present in no other disease. Then pure cultures 
must be made of the -organism and some animal in which it will grow 
must be inoculated with this pure culture. The same lesions must 
be produced in the inoculated animal as those from which the origi- 
nal organism was procured. And finally, the same variety of organism 
with which the investigation started must be obtained from the 
inoculated animal. 
In what way do bacteria gain access to the body ? 
Through the skin, the alimentary tract and the respiratory organs. 
