PLAGUE 161 has little influence on plague. It may flourish in alluvial deltas, on calcareous ridges or granitic mountains. Moisture in the soil has generally been thought to be an important factor in its production, but, though often found in marsh situations, such as the banks of the Nile, the Euphrates, or even the Volga, it also occurs in India at elevations approaching 7000 feet, and in Kurdistan at 5000 to 6000 feet above the sea. The temperature most favourable to plague is a moder ately high one. The disease is unknown in the tropics. When prevalent in Egypt it was said never to penetrate farther south than Assouan. It has not crossed the plains of India within historic times. Where the disease does occur, a temperature of 80 to 85 or more, combined with absence of moisture, usually stops the epidemic. In Egypt it was observed to cease as an epidemic almost suddenly about the 22d to 24th of June, and not to begin again till September. In Irak it dies out suddenly during the summer. When the temperature rises above 86 it begins to diminish; and it ceases abruptly at a temperature of 113. In India it has been observed by l)r Francis when the temperature of his tent was 83 to 95, or in a grass hut to 105, while the air was moist ; but he thinks a lower temperature with dryness renders the poison inert. On the other hand, in northern countries, the disease is usually checked by the cold of winter, starts up in the spring, and is most active in August and September. To this rule there have been some remarkable exceptions, such as the epidemic on the Volga in 1878-79, which raged during severe winter weather, and the great plague of Moscow in 1770. Sanitary Conditions. Of all the co-operating causes of the plague, undeanliness is the most powerful, meaning by this the accumulation of decaying animal matter around human bodies or dwellings. The saturation of the soil with filth is perhaps the most important point. A plague seat in Mesopotamia is thus described by Colvill : " The ground is so saturated with moisture that the refuse of the village is neither absorbed nor evaporated, but . . . acquires the form of a bluish-black oily fluid, which sur rounds the huts and covers the paths, and stains the walls 2 feet from the ground ; and in fact the village is in such a state of filth that it requires to be seen to be believed." Of the people among whom the Pali plague of India raged it is said " they were filthy beyond conception " (Francis). There can be little doubt that European cities in the Middle Ages, and down to the 17th century, presented very similar conditions. These conditions may be con sidered to act by supplying a suitable environment for the life and growth of the organized poison (or bacterium) outside the human body. Where these are wanting one of the main factors in the spread and permanence of the disease will be absent, a fact which makes it probable that increased cleanliness is the chief cause of the disappearance of plague from Europe. Overcrowded dwellings, especially with deficient ventila tion, greatly favour the spread of the disease ; but this is not necessarily correlative with density of population, and plague may flourish in thinly peopled countries. Of social conditions poverty has by far the most power ful influence on the spread and development of plague. Many plague epidemics have followed on years of famine, or been connected with destruction of crops and cattle. The races among which the disease is endemic are almost without exception under-nourished, if not destitute. In the villages on the Volga there appeared to the writer, in 1879, to be little destitution, though the diet of the people was very meagre. In all city epidemics the poor are the chief or almost the only sufferers. This is as true of Baghdad in the 19th century as it was of London in the 17th. Those of the upper classes who have been attacked have been chiefly doctors, clergy, officials, and others whose occupations take them among the sick. Origin and Spread of Plague. Although the above- mentioned conditions are those in which plague originates, and may be considered in a general way essential to its continued existence, it is plain that they do not account for its origin. Poverty, overcrowding, filth, and marsh soil, with a temperature suited to plague, occur in many parts of the world where this disease has never been heard of or has ceased to exist. The geographical distribution of diseases cannot, any more than the distribution of plants or animals, be explained by climatic causes ajone. With regard to plague it is quite clear that there are some parts of the world where it is at home, or, as the phrase is, " endemic. " In other parts it is probable (or, as some think, certain) that its existence, and even its periodical recur rence, depend on importation from an endemic centre. Although it is not always easy to distinguish between these cases, they must be considered separately. In the case of an endemic disease we suppose that the poison is either kept in existence by continued transmission from one case to another, or that it can subsist outside the human body in soil, water, or otherwise. The first mode of existence is that of a pure contagious disease, such as small-pox ; and it is plain that this mode of continued existence obtains in the case of plague also. It is not, however, clear that the second may not also be one of the modes of existence of plague, which would then be a so- called "miasmatic" disease like ague, as well as a conta gious one. In India, for instance, the disease appears as if it depended on a poison in the soil, since it returns years after to the same spot, appearing in many villages simul taneously ; and some morbid influence causes the death of animals (rats) which live under ground. Similar facts have been observed in China ; and, if further inquiry should confirm the hypothesis, it would show that plague is (like anthrax or the " steppe murrain " of cattle) both miasmatic and contagious. If so, there is no difficulty in supposing the disease to be carried by contagion to a distant part, and there to be established in the soil, for a longer or shorter period, as the conditions are more or less favourable. The adoption of this hypothesis would remove many of the difficulties attending the explanation of plague epidemics, and to some extent reconcile the contro versies of the last three centuries between the " contagi- onist " and " non-contagionist " schools. It has been maintained by the former that European epidemics have always been caused by the importation of the disease from its home in the East, by the latter that it arose on European soil in the same way as in Egypt or Syria. In the case of an imported non-endemic disease, the only question which arises is how the importation is effected, whether the disease may be brought by the air alone, whether by infected persons only, or whether also by objects which have been in close relation to infected persons. Transmission of the disease by the air cannot be pronounced impossible : and there are facts to show that it is even probable with distances measured by yards, or possibly even hundreds of yards; but there is no evidence whatever that the disease has ever been carried by the air over distances measured by miles. Transmission of the disease by infected persons over longer or shorter distances, and from one country to another, is an established fact. Transmission by infected objects over great distances and from one country to another seems less clearly established. The last two cases must be separately considered. 1. It is clear that the first necessary condition to such transmission is contagion, or transference of the disease from the sick to the healthy. The existence of contagion
XIX. 21