We do not ignore the fact that small-pox, like other similar diseases having an epidemic character, may be absent for a length of time from certain districts and then break out again; nor that each epidemic has its period of increment and decrement, and varies in its degree of malignancy. But a full and careful review of the whole history of small-pox since the introduction of vaccination, proves to every unprejudiced mind that every recurring epidemic finds its victims, with comparatively few exceptions, among the unvaccinated, that its spread is arrested by renewed attention to vaccination and its vigorous enforcement, and that, even taking into account the countries and localities where from various causes it has been neglected, the mortality from this foul and fatal disease, small-pox, has been enormously reduced. Human lives have been saved, and human life prolonged to such an extent that it is impossible to estimate the benefits that mankind has derived from the genius and devoted patriotic labours of one man.
That doubts and difficulties in connection with this subject, involving the well-being of the whole human race, have lately arisen, must be admitted. But there is good reason to believe that, by modern researches on the subject of epidemic diseases and the germ theory of disease, these doubts are already being dispelled, and that the difficulties will be speedily obviated.
The grounds for this belief will be understood by the consideration of those scientific investigations to a brief detail of which we now proceed. The reader will then also be better able to judge of the propriety, and necessity of certain measures which, to the uninformed, must appear objectionable or even repulsive and arbitrary.
We now, then, turn to the remarkable experiments and discoveries of M. Pasteur, which have gained for him a world-wide reputation, and the bearing of which on the science of preventive medicine is commanding the attention and admiration of the whole scientific world, and indeed we may say of mankind at large. M. Pasteur is not a medical man, nor, indeed, a physiologist. He is simply a French chemist, a modest, retiring labourer in the field of science whose sole object has been the discovery of truth, and whose chemico-physical researches gained for him the Rumford Medal of the Royal Society in 1856. Having devoted himself specially to the chemistry of organic substances, he was naturally attracted by the discovery of Cagniard de la Tour, that yeast is really a plant, a species of fungus, whose vegetative action in fermentable liquids is the true cause of their fermentation. This was so opposed to the theories of all the chemists of the day, among whom may particularly be mentioned the celebrated Liebig, that it met with their warm opposition. When, however, Helmholtz and others succeeded in showing that by preventing the passage of the minute organisms constituting the yeast plant into fermentable liquids, no fermentation took place, the doctrine soon became established that the first step in the process of alcoholic fermentation is due, not to ordinary chemical changes, but to the presence of living organisms. In like manner the putrefaction and decomposition of various liquids containing organic matter was found to be due, not to the simple action of the oxygen of the atmosphere, but to the introduction from without of microscopic germs which found material for their development in such liquids. So that if by mechanical filtration of the air the entrance of such germs can be prevented, or if by heat or other means they can be destroyed, any fluid, however readily it may undergo putrefaction in ordinary circumstances, will remain perfectly sweet, though freely exposed to the air. And the same fluid will undergo a different kind of fermentation according as it is subjected to the action of different species of germs. These and other facts of scarcely less importance, which cannot here be detailed, induced Pasteur to test the application of the doctrines deduced from them to the study of disease in living animals.