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THE PRESIDENTIAL ADDRESS. #3 
either a contaminated water supply or a faulty drainage system, and the 
municipal authorities ought to be called to account. In England, owing 
to improved sanitation, cases of typhoid fever are fifteen times less than 7 
they were fifty years ago. 
But it is not always possible to ensure good hygienic surroundings—for 
example, among troops on active service. It is therefore legitimate under 
certain conditions, and especially in time of war, to practise a less sound, a 
less fundamental, method of prevention, and this second method is known 
as inoculation or vaccination. 
In order to understand how this acts, let us consider, for a moment, 
what takes place in a man’s body when he is attacked by the typhoid 
bacillus. Everybody knows that the bacillus gives rise to poisons or toxins 
which cause the fever and other symptoms. But the cells and tissues of 
the man are not passive under the attack. They at once begin to fight 
against the infection, by forming substances in the blood to neutralise these 
toxins, hence called antitoxins or antibodies, and their function is finally 
to destroy the invading germs. If the man recovers he is immune from a 
further attack by the presence of these antibodies in his blood. He has 
become immune by passing through an attack of the disease. 
This is the foundation of the second way of preventing infectious 
diseases. Speaking broadly, it means that you subject a man to a mild 
attack of the fever in order that his blood and tissues will respond to the - 
stimulus by producing antibodies. 
This method takes its origin and name from that of vaccination against 
smallpox. Jenner solved that problem by the accidental discovery of 
vaccinia, a form of smallpox attenuated or weakened by passage through 
another species of animal. This weakening of the virulence of a micro- 
organism by passage through another kind of animal is by no means 
uncommon in nature. 
Pasteur, following on these lines, conceived the idea of weakening or 
attenuating the virulence of the living bacilli by artificial means, so as to 
give rise to a mild attack of the disease, and in this way to render 
animals immune. This he did with marked success in anthrax and chicken 
cholera. 
The next forward step in this method of preventing disease was made 
by Haffkine, a pupil of Pasteur, who about the year 1894 produced a 
vaccine against cholera, and a few years later another, against plague. 
In the course of this work it was discovered that it was not necessary 
to use living cultures of the bacilli, but that vaccines made up of dead 
