30 
BACTERIOLOGY. 
Factors 
influenc¬ 
ing in¬ 
fection 
organism is called infectious. As examples of infec¬ 
tious disease, diphtheria, pneumonia, influenza, tuber¬ 
culosis, and syphilis may be mentioned; although there 
are many others. A contagious disease is one that is 
transmitted from one person to another by simply 
coming into the presence of or touching the sick. 
Smallpox, scarlet fever, measles, chickenpox, and Ger¬ 
man measles are usually classed as contagious dis¬ 
eases. All contagious diseases are infectious, but not 
all infectious diseases are contagious. Diseases like 
cholera, glanders, pneumonia, plague, tuberculosis, 
and syphilis cannot be transmitted through the air or 
by coming into the presence of the sick. Typhoid 
fever may be considered infectious through water and 
other infected foods, and contagious by contact with 
the so-called typhoid carriers. 
The terms “infestation” or “infestion” are applied 
to diseases caused by entrance into the body of small 
parasites such as amebse, worms, and so on. 
While the presence of pathogenic bacteria is 
necessary to cause infection, other factors of much im¬ 
portance must be taken into consideration. This must 
be so as everyday experience shows. In any epidemic 
of infectious disease only a portion of those exposed 
become infected. Even among those infected the dis¬ 
ease presents all variations from the very mild to the 
most severe. The factors that influence the onset and 
course of infections relate both to the bacteria and the 
individuals exposed to them. 
So far as the bacteria themselves are concerned, 
