20 THE PRESIDENTIAL ADDRESS. 
In some of them there is no attempt made at prevention, except that the 
sick are isolated and placed under quarantine for a longer or shorter period. 
But in others there are well-known methods of prevention even when 
the virus is quite unknown. The best example is smallpox, the ravages 
of which have been completely held in check since the memorable discovery 
of Jenner. As has already been argued, this method of prevention, by 
inducing a mild or attenuated form of the disease, is at best a clumsy one, 
and when the natural history of the smallpox virus is better known it 
may be hoped that a more fundamental method of preventing this disease 
may be discovered. In the meantime the best means at our disposal is 
by the use of vaccine lymph, and people should recognise their responsi- 
bility to the community if through ignorance or selfishness they refuse 
to have their children vaccinated. 
Another well-known disease with an unknown virus, rabies or hydro- 
phobia, has also, by the genius and intuition of Pasteur, been robbed of many 
of its terrors. The mortality following bites of rabid animals has fallen 
from 16 per cent. to less than | per cent. 
But in rabies, when the conditions are favourable, the radical method 
is to drive the disease altogether out of the country by the careful adminis- 
tration of muzzling and quarantine laws. This was carried out successfully 
in England at the beginning of the century. 
Trench Fever. 
There are among the diseases of undetermined origin a few which are 
slowly emerging from the unknown into the known. One of the most 
interesting of these is trench fever, which came into great prominence 
during the war. 
The history of the investigation of this fever is interesting, and 
well illustrates the method of studying a disease with a view to its 
prevention. 
Before the war, trench fever was unknown, though there is some 
evidence that it had been recognised at an earlier date in Poland and called 
Wolhynia fever. Be that as it may, it is quite certain that, though it was 
unknown on the Western Front at the beginning of the war, it is no 
exaggeration to say that it became one of the most powerful factors in 
reducing our man-power, probably more than a million cases occurring 
among the Allies on the Western Front. In 1917 in the Second British 
Army alone, out of a total of 106,000 admissions to hospital at least 
20,000 of the cases were trench fever. 
