CHAPTER IV 
THE STRUGGLE BETWEEN BACTERIA AND THE 
BODY INFECTION AND IMMUNITY 
I. Infection 
By infection we understand more than mere entrance 
of bacteria to the animal body; an infection means the 
entrance and successful multiplication of bacteria in 
the animal body, with the subsequent development of 
a disease (which is then called infectious disease). 
That the mere presence of bacteria in the animal body 
does not mean infection or infectious disease is obvious 
from the fact that several species of bacteria are con¬ 
stantly present in certain parts of the body; the colon 
bacilli are always present in the intestines, the bacillus 
xerosis is very frequently found in the lining of the 
eyelids (conjunctiva), staphylococci, streptococci, and 
pneumococci are very often found in normal mouths. 
In order that the infection should take place the 
following conditions should be fulfilled: 
1. Bacteria should gain entrance to the body by a 
path specially adapted to their requirements; thus, cer¬ 
tain bacteria invariably attack through the gastroin¬ 
testinal tract, e. g., the typhoid bacillus and cholera 
spirillum; some bacteria enter the body only through 
the skin, e. g., staphylococci, streptococci, tetanus bacil¬ 
lus, etc. These different paths are so necessary for 
their successful invasion that should the typhoid bacil¬ 
lus (w r hich is a gastrointestinal invader) be rubbed into 
the skin, or, conversely, should the staphylococcus 
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