CHAPTER XV 
EPIDEMIOLOGY 
No attempt will be made here to present a complete discussion of 
the subject, but merely a few remarks about epidemics and the method 
of investigating them. Boudreau (1914) has defined an epidemic as an 
unusual number of cases of a communicable disease, arising within a 
short period, in a limited area, and traceable to a common source or 
sources. The food bacteriologist may often find it necessary to inves- 
tigate the cause of an epidemic either to protect an innocent article of 
food or to check the spread of infection. It is by no means an easy task 
in some epidemics to pick out the real cause; in others, by using a logical 
procedure, the epidemiologist may select with great accuracy the focus 
of the infection and see the reward of his efforts by the immediate 
decrease of the number of cases. He must be on his guard for any and 
all information which may help him towards his goal. 
Different methods have been used in the past by epidemiologists. 
It used to be customary for him to use indirect methods in securing his 
data. If called to a community where there was an unusual amount of 
disease, he would go equipped with sample bottles for the collection of 
specimens of milk and water and would spend much time in the early 
part of his investigation in inspecting water supplies, methods of sewage 
treatment, etc. The mode of attack is now quite different and marks 
one of the distinct advances which preventive medicine has made. 
The present practice in investigating epidemics has been well stated by 
Hill (1912). A certain amount of information is always necessary with 
regard to the characteristics of the disease and some of the different 
methods of spreading. Certain characteristics have been worked out for 
epidemics caused by food, water, carriers, etc. These are by no means 
constant but serve in a general way along with such other general data 
such as, population of the community, endemicity of the disease in the 
community, general appearance, etc. If this preliminary, general 
study 1s well made the epidemiologist may save much time when he 
comes to analyze his other data because suggested causes of the infec- 
tion may be quickly eliminated. 
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