EPIDEMTCS_, PAST AND PEESENT. 595 
fevei% Oriental plague^ yellow fever^ diarrlioea^ typhoid or 
enteric fever^ cholera^ dysentery^ agne and remittent fevers^ 
influenza^ the sweating sickness^ and the dancing mania. 
1. Small-joox, the most loathsome of all diseases^ is believed 
to have prevailed in India and China from time imme- 
morial. About the middle of the sixth century it is supposed 
to have been conveyed by trading vessels from India to Arabia^ 
and the Arabian army at the siege of Mecca^ in the year 569^ 
was the first victim of its fury. From Arabia it was imported 
into Europe by the Saracens^ and there is evidence of its 
existence in Britain before the ninth century. Before the in- 
troduction of vaccination^ small-pox was one of the chief 
causes of mortality in all the countries where it prevailed^ and 
even now it occupies a prominent place in our mortuary returns. 
During the twenty-four years 1838-61^ 125^352 of the popula- 
tion of England and Wales, and 21,369 of the population of 
London, died of small-pox ; or, in other words, 1 in 75 of the 
total deaths in England and Wales, and 1 in 63 of the total 
deaths in London, were due to this disease. Small-pox is not 
confined to any race or quarter of the globe. At the present 
day, its appearance can, in the great majority of instances, be 
traced to contagion. It is evident, however, that it must 
at one time have had an origin, and it is reasonable to 
infer that what happened once may happen again. Small-pox 
is known to attack many of the lower animals as well as man, 
and there are grounds for believing that ifc originated amoug 
the former, and by them was communicated to the human 
species. A careful study of epizootics — our ignorance of 
which has been disclosed by the present cattle plague — may 
ultimately reveal the mode of origin of the poison of small- 
pox. The disease varies greatly in its prevalence at different 
times. In other words, it is sometimes epidemic, at others 
not. Some of these epidemics are local ; others are widely 
extended. All exhibit a gradual rise, culmination, and decline, 
the decline being always less rapid than the advance. It is 
difficult to account for the occurrence of these epidemics. 
They are independent of hygienic defects, season, temperature, 
or any meteorological conditions of which we are cognizant. 
They are probably due to causes tending to depress the general 
health of the population, and so to predispose it to the action 
of the poison. For nearly two centuries it has been a common 
observation that epidemics bf small-pox have co-existed with 
epidemics of other contagious diseases. The gradual accumu- 
lation also in a district, of unprotected persons, owing* to the 
neglect of vaccination, will also predispose to the occurrence 
of an epidemic, after the introduction of the poison. In fact, 
to the neglect, or careless performance of vaccination, is 
