593 
EPIDEMICS, PAST AND PEESENT— THEIR ORIGIN 
AND DISTRIBUTION. 
»o« 
E pidemics, derived from the two Greek words en)., 
among, and ^rifxog, people, are those diseases which for 
a time prevail widely among the people of any country or 
locality, and then, for a longer or shorter period, either 
entirely, or for the most part, disappear. There are few 
diseases to which the human race is liable that may not, 
under favourable circumstances, take on the epidemic form. 
For example, diseases of the organs of respiration are very 
apt to become epidemic, in seasons characterized by extreme 
coldness or dampness of the atmosphere, or by great and 
sudden alternations of temperature. In a strict sense, how- 
ever, the term epidemics is not usually employed in reference 
to the diseases of individual organs of the body, but is 
restricted to those derangements of the entire system de- 
pending upon the absorption of some poison, or the action 
of some ^Gnfluence,^^ from without. In the latter class of 
maladies, the individual organs may become diseased, and 
the derangement of their functions may modify the symptoms 
resulting from the primary poison or ^ influence but then 
the local diseases are the secondary result of the general 
disorder of the constitution, and not the source and origin of 
all the mischief. 
Some epidemic diseases possess the power of self-propaga- 
tion, that is to say, the poison or influence may be com- 
municated by infected persons to persons in health, and the 
disease is then said to be contagious,* while others are entirely 
destitute of any such property. Scarlet fever and small-pox 
are familiar examples of the former class ; ague and influenza, 
of the latter. 
It is still a vexed question whether a disease that is capable 
* The terms “ contagion ” and “ contagions ” are here used in their widest 
signification, and are ap2fiied in this essay to ail diseases caj^ahle of })r0' 
jDagation by infected individuals to persons in health. 
