The foregoing facts are quite sufficient to make us realize both the possibility and the danger of a world-epidemic; a danger which has existed for some years and which recently has been especially menacing to the United States.

Causation of the Disease.—Plague is an acute infectious epizoÖtic disease, caused solely by Bacillus pestis, a bacterial organism. The disease is common to man and to a number of the lower animals and fowls.

Prominent among the animals susceptible to the disease is the rat, and from this animal, through the intermediation of the flea, by far the most cases of human plague arise. In California the ground squirrel (Citellus beecheyi), a rodent closely related to the marmots of Asia, plays a similar rÔle. Of the Asian marmots, the tarbagan, a large rodent, also commonly suffers from subacute chronic plague, which is transmissible to man as an acute disease by the fleas which the animal harbors.

Its Conveyance.—Although conveyance of plague through rats by contact alone—that is to say without the medium of the flea—is denied by modern experimenters, it is perhaps wiser and safer to consider the disease infectious, inoculable and contagious in the common medical meaning of these terms. While it is usually conveyed to man by the flea, it may be acquired by the inhalation of plague bacilli and, according to some authorities, by ingesting or swallowing the bacilli.

When infection takes place through the digestive tract, or in other words, by the ingestion of bacilli, either the flesh of plague-infected animals or fowls, or food superficially contaminated with plague bacilli by rats, cockroaches or other carriers, serves as the medium.

Speaking practically, the possibility of infection through ingestion is nearly negligible. Indeed, the conclusion of Simpson in regard to this possibility has been disputed and denied. However, the recent occurrence of plague in a cat in Manila, in my own experience, observed with me and carefully worked out by Dr. Otto SchÖbl, points strongly to the possibility of ingestion plague, the cat in this case apparently having acquired plague from eating rats dead from plague.A full account of this case appears in the bacteriologic observations of Dr. SchÖbl and in my recital of the history of the Manila epidemic.

Types of Plague.—Plague in man may be of several types and these are designated by names descriptive of the symptoms or of the regions of the body most affected. Thus we have bubonic, septicÆmic and pneumonic types. As both mild and virulent cases occur, we also use terms descriptive of the severity and course of the cases. Thus we describe certain cases as ambulant, abortive, larval and fulminant. In the rat the evidences of plague are less striking in life than they are at the post-mortem table. Indeed plague-stricken rats, either naturally or artificially (experimentally) infected, often show very slight evidences of disease before death. Chronic plague in rats and a relative immunity to inoculation in certain wild rats are fairly well recognized phenomena.

Flea Conveyance of Plague Bacilli.—Both male and female fleas convey plague, but the exact method of carrying the plague bacilli from diseased rats to man, while fairly well determined, is of such recent decision as to leave room for further experimentation. At present it is believed that the flea deposits plague bacilli, at the time of biting, upon the skin, by ejecting the contents of its rectum and by regurgitation of its stomach contents. At least the flea is known to perform these acts at the time of biting, and the rubbing or scratching of the flea bite with the hand may easily introduce the bacilli into the skin at this spot.[1]

[1] Acknowledgment is hereby made to the Contributors to "The Rat and Its Relation to Public Health" by various authors, prepared by direction of the Surgeon-General, P.H. and M.H.S., for numerous facts utilized in the preparation of this article. The particular contributors whose valuable chapters have been drawn upon for information are D.E. Lantz, C.W. McCoy, D.H. Currie, Carrol Fox, Rupert Blue, W.C. Rucker, R.H. Creel, M.J. Rosenau, V.C. Heiser, W.C. Hobdy, and J.W. Kerr.

The possibility that the flea introduces the plague bacilli upon his mandibles, or the skin-piercing armament with which he is provided, is also to be considered. However, the following facts support the first proposition. It has been experimentally shown that the average capacity of a flea's stomach is about one-half of a cubic millimetre and that thousands of plague bacilli may be ingested by the flea during the biting of a plague-diseased rat; that the plague bacilli multiply enormously and for many days in the flea's stomach and that the bacilli are found only in the insect's digestive tract; that plague bacilli are regurgitated from the stomach and are voided from the rectum with the digested blood.

It has also been proved that almost all varieties of rat fleas, under favorable circumstances, will bite man and that the most common human flea (Pulex irritans) is frequently found upon rats, the flea, generally speaking, being much less particular in his choice of hosts and in his permanence of residence than most insects and ectoparasites in general.

Of the rat fleas, Pulex pallidus (Loemopsylla cheopis) is common under various names in India, the Philippines, Australia, Italy, Brazil and in tropical countries generally. It bites both rat and man. Ceratophyllus fasciatus, the common rat flea of Great Britain and the United States, also bites both rat and man. In North America and elsewhere certain other fleas of the genus Ceratophyllus have been found upon ground squirrels, cats, rats, sparrows and in chicken yards.

Dog fleas and cat fleas (genus Ctenocephalus) also infest rats, and fleas of other genera are found upon mice, rats and ground squirrels rather indiscriminately.The significance of these facts in connection with prevention of plague is apparent and it is plain that our warfare against fleas must be made upon all fleas and not upon a single variety. In this connection the possibilities of the conveyance of plague bacilli by other suctorial parasites and by insects which are not parasites, must be borne in mind.

Thus the bed-bug, the louse, the tick and the mosquito must be suspected as possible intermediaries and the fly and the cockroach as possible food contaminators. Indeed, laboratory experiments have already incriminated bed-bugs, flies and lice as potential vectors of plague bacilli.

Experiment and observation have demonstrated, however, that above all other parasites and insects, the flea is most likely to convey the plague germ from rat to man, by reason of his frequent excursions from rat-host to human-host, his taste for blood from either host, his enormous activity and his ability to jump. After a searching inquiry into the plague question the Indian Plague Commission came to the conclusion that contagion plays a very minor part in the spread of the disease, less than three per cent of human cases being so acquired.This commission also decided that infection is conveyed from rat to rat and from rat to man solely through the agency of fleas. While these conclusions are probably true—and therefore of the utmost importance from the standpoint of practical prevention—I should question whether the other possibilities, however remote, are entirely negligible.

Seasonal conditions may affect the course of an epidemic in various ways. (a) By effect upon flea prevalence, cold weather greatly lessening the number of insects. (b) By effect upon rats, cold weather and rains either driving them from overground to underground, or vice versa, or from their principal avenues of travel in cities (the sewers), into houses and buildings. (c) By effect upon the plague germ, Bacillus pestis. The resistance of this organism is very variable, sunlight and drying being its greatest enemies, while darkness and dampness are its chief allies. So far as temperature is concerned, the plague bacillus is not likely to be seriously affected by natural temperatures, as it is not destroyed by heat below 150 degrees Fahrenheit, nor by cold measured by zero Fahrenheit, which means that it survives freezing, generally speaking.

It is probable that the periods of greatest seasonal prevalence of plague will be found to correspond generally with increased prevalence of rat fleas. During the periods when rat fleas are absent or least prevalent, the disease is perpetuated in the form of chronic (subacute) rat plague in a small number of the rodents. The India Plague Commission made and verified this observation.

Cholera epidemics often abate spontaneously and this is believed to be due in part to attenuations of virulence and changes in the cholera organism which may be demonstrated in the laboratory. We can hardly hope for such spontaneous abatements in plague epidemics, as it has been found difficult to attenuate or to intensify cultures of plague bacilli permanently in laboratory experiments with animals. If it is true that plague epidemics are often marked by a preponderance of mild cases in the early days and a gradual subsidence of intensity of the cases as the epidemics wane, we probably will have to look to the susceptibility of our patients for our explanation of this phenomenon, rather than to variations in the virulence of the plague bacilli. If plague bacilli continue to be distributed to susceptible people the disease should continue with a general stability of virulence.

Stability of Virulence of B. Pestis.—According to Strong, stability of virulence is a marked characteristic of B. pestis, it having been shown by him that it is difficult to increase the virulence of a very virulent strain or to intensify an attenuated one in laboratory animals, working with monkeys, rats and guinea-pigs.[2] If his observations are correct (and they seem to correspond with the findings of other observers), the oft-recorded occurrence of a preponderance of mild cases of plague in the early days of an epidemic and the gradual subsidence in intensity of the disease as the epidemic approaches its close will have to be explained upon other grounds than those of variability of virulence by attenuation of virulent strains alone. While he admits that B. pestis may become attenuated under certain conditions many times during the course of an epidemic, it may also regain its virulence, he contends, under other conditions.

[2] "Studies in Plague Immunity," R.P. Strong, Philippines Journal of Science, June 1907, No. 3. Frequent reference has been made to these studies in the preparation of this article, for which acknowledgment is hereby made.

With these facts concerning the cause and the manner of extension of plague and its menace before us, we are in position to approach the problem of prevention intelligently, and in the case of plague prevention is preËminently preferable to cure, as well as decidedly more practicable.

I think we may be permitted here to sum up the problem of plague prevention thus: Without fleas, without rats, or without human plague cases, there can be no extension of plague, practically speaking.

Therefore the destruction of both rats and fleas, the isolation of human plague cases, and the exclusion from them of all suctorial parasites and insects, will provide practical security for mankind generally.

A word concerning pneumonic plague may be permissible. This form of plague occasionally occurs in epidemics of great fatality, as, for example, the epidemic in Manchuria, North China, a few years ago.The mystery of this outbreak was largely dispelled by the work of the Americans, Strong, Teague and Barber, of the Bureau of Science of Manila.

The occurrence of secondary pneumonia in bubonic or septicÆmic plague is rather common and it is likely that such secondary plague pneumonias are the starting points of epidemics of pneumonic plague, i.e., of cases of primary plague pneumonia, the point of infection being in the respiratory organs and the infection being acquired through the inspiration of plague bacilli.

The principal prerequisites seem to be an extremely moist atmosphere under confined conditions and a low temperature; conditions most unfavorable to evaporation and ventilation. Under these conditions the pneumonic patient sprays plague bacilli into the air while coughing and droplet infection follows.

It is therefore apparent that epidemic pneumonic plague is controllable by sanitary and hygienic measures and, furthermore, that in the absence of original cases of bubonic and septicÆmic plague, with secondary plague pneumonias which give rise to primary plague pneumonia in the manner explained, respiratory plague in epidemic form will not occur.

There is no evidence pointing to the conveyance of respiratory plague by insects or other carriers.