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The secretions and excretions of plague infected individuals themselves offer innumerable chances for indirect infection. The so called "Drop Infection," with the exception of severe pneumonic types of epidemics, would appear to play but a small part in the spread of the disease.
The links in the chain of infection in plague may be named the following:-
(1.) The source of infection.
(2.) The method of transportation.
(3.) The gates of entrance.
(4.) The amount of the virus.
(5.) The quality of the virus.
(6.) The susceptibility of the individual.
If one of these links is missing there is no chance of the disease breaking out. A priori it is of little consequence for prophylaxis what link in the chain is at- tacked, provided the causative agent in the particular link is effectively combated. For general purposes, however, it would seem best to attack each link in the chain simultaneously, provided an opportunity presents itself.
The international arrangements and quarantine regulations, regarding the prevention of an exotic disease like plague, have become extremely complicated and rigid within the past few years.
Exotic diseases, in general, are not like measles, tuberculosis, venereal diseases, etc., which are endemic; they are always introduced into a country.
They are never of autochthonous origin. Another point of interest here is that these exotic diseases, namely, plague, cholera, and probably yellow fever, are not only introduced into a country, but the method of introduction or the avenue of infection is almost always by way of the sea. Maritime commerce is therefore largely responsible for the spread of plague, and the defensive measures adopted against these diseases have much in common. The measures directed against the spread of cholera are almost identical with those laid down to combat the introduction of plague.
Within recent years, another channel of infection has been opened up for the spread of the disease. Railways are being constructed in all parts of the Con- tinents of Europe and Asia, and will constitute an element of even greater danger than the sea. The disease will, in this way, be rapidly transported from one end of the world to the other, and all the skill and vigilance of sanitary authorities will be required for the prevention of its introduction. The construction of the Trans-Siberian, the Bagdad, the Russo-Indian, the Russo-Persian as well as numer- ous railways in plague stricken Southern China will add to the already over-taxed energies of sanitary authorities and public bodies. The present day regulations and international quarantine arrangements in order to insure a trustworthy pro- phylaxis are widely known to all those interested in the prevention of exotic diseases.
What remains, is to consider these quarantine measures critically and judge whether such regulations and arrangements in their present form have fulfilled our hopes and insured us freedom from plague or other similar diseases.
The answer must be in the negative.
In fact, one may go further and apart from certain exceptions, assert that with our present state of knowledge, we possess no means of being absolutely certain as to the prevention of the introduction of plague into any particular country or city.
The grounds for such a sweeping statement are as follows: The very essence of quarantine directs its energies in the first place to plague patients, in the second place to those individuals in whom the disease is inoculating and thirdly to the possibility of indirect infection through merchandise and personal effects. How- ever, our ever advancing knowledge of plague has shown, only within the last few years, that two other factors, hitherto disregar led, have a very important bearing on the whole question.
These are the knowledge of the existence of latent cases of plaque, and the predominant role played by rats.