CHAPTER IV. 
INFECTION, IMMUNITY AND IMMUNITY 
REACTIONS. 
In the preceding chapters we have been dealing 
with the subject of bacteriology in the broadest sense. 
Attention has been directed to the function of bacteria 
in the life of the world; to their appearance, their 
manner of growth, and the means employed for their 
destruction. As physicians and nurses our interest 
centers about a very small part of the bacterial king¬ 
dom, the one having to do with the production of 
disease. Bacteria that produce disease are termed 
pathogenic, while those varieties that do not are 
called non-pathogenic. By far the larger number of 
pathogenic bacteria thrive only in the living tissues 
of animals. These are called parasites. Some kinds 
of bacteria thrive only on dead tissues or wounded 
surfaces and, by decomposing them, form poisons 
(ptomaines) which may be absorbed and give rise to 
symptoms such as fever, chills, and headache. These 
are termed saprophytes. When pathogenic bacteria 
gain access to the tissues and produce injury and 
symptoms, we say that infection has taken place. 
Here it may be well to' say a word as to the mean¬ 
ing of the terms “infectious” and “contagious.” They 
have been used somewhat loosely and have led to a 
great deal of confusion. Any disease that is caused 
by the entrance into the body of a living micro- 
(29) 
Patho¬ 
genic and 
non-patho¬ 
genic bac¬ 
teria 
Infection 
defined 
Infec¬ 
tious and 
con¬ 
tagious 
