32 
BACTERIOLOGY. 
Infec¬ 
tion from 
bacteria 
outside 
the body 
Infec¬ 
tion from 
bacteria 
living 
inside 
the body 
organism to resist infection will be discussed more 
fully under the subject of immunity. 
How does infection take place? It is the result of 
the invasion of the body tissues by pathogenic bacteria 
that live either on the surface of the body or from 
those that live on the mucous membranes inside the 
body. Injuries play an important part in causing 
infections. Injuries caused by firearms may be the 
entering point of tetanus bacilli, the cause of lockjaw, 
while rabies or hydrophobia is spread through the bites 
of mad dogs. Careless manipulations with soiled 
catheters, speculums, syringes, and so on may cause 
injury to the tissues and be the means of introducing 
bacteria. In the case of the contagious fevers like 
measles, chicken-pox, whooping-cough, and scarlet 
fever the infecting agent seems to be in the air and 
causes infection by being inhaled. Bedding, clothing, 
and utensils that have been contaminated with infec¬ 
tious material may be the means of spreading infec¬ 
tion. Finally, the bites of insects and vermin may 
cause infection. It is known that certain kinds of 
mosquitoes transmit malarial fever and yellow fever; 
flies may spread typhoid fever by depositing the 
typhoid bacilli on food materials. 
The body may be looked upon as the host for 
large numbers of bacteria. At birth, however, all 
healthy animals are free from bacteria; but almost 
immediately afterward they are deposited upon the 
surface of the body by the dust in the air, and are 
introduced into the body by food and by the air 
