diffused or "spreads" from one place to another, and that its spread is connected mediately or immediately, in most cases at least, with human intercourse. But this diffusion appears to take place as a rule slowly, and to be effected by the formation of new foci of contaminated atmosphere. Such foci on land will be inhabited houses, and the disease will creep in a gradual though irregular manner from house to house and street to street. It was so in London in 1665 and in Russia in 1878, as has been said, the disease was confined to one village for two months, though for great part of the time communication was perfectly open. In 1834 plague existed eight months at Alexandria before passing to Damietta and Mansoorah, though traffic was quite uninterrupted. These new foci of disease are doubt less mostly produced by persons infected with the disease, actually or in incubation, who form a contaminated atmo sphere around them in a place previously healthy. Transmission of the disease by sea may take place in the same manner, a ship forming a focus of disease as easily as a house, and being obviously specially liable to concen trate the poison. It is by a floating atmosphere of plague, and not by casual contaminated objects, that the disease has been conveyed, when it has been, from one port to another of the Mediterranean. The reality of the mode of transmission is shown by the fact that between 1720 and 1846 twenty-five ships arrived at French and Italian ports with the plague among their crews ; and in the case of those arriving at Marseilles (ten in number), which were carefully observed, there were several instances of plague being communicated in the lazaretto to surgeons and others, or to those placed in charge of the ships. Of these persons several died, without, however, any exten sion of the disease to the town. From this it is clear that plague may be transmitted by ships, and may spread at the point to which it is conveyed, if the surrounding cir cumstances are favourable. In all these cases the ships had left the infected ports at a time when an epidemic of plague, and not merely sporadic cases, prevailed there. No similar facts are on record as to the importation of plague by ships to England, the probable cause of this difference being the greater length of the voyage from the Levantine ports, and the precautions taken at those ports to prevent the shipment of infected persons or goods. Plague has never been brought to an English quarantine station. 1 In such cases it must remain undetermined whether the disease would have spread, had it not been interrupted by the quarantine. As we have seen, plague will often die out in the cases which convey it without spreading ; and hence some have supposed (with Sydenham) that an " epidemic constitution " is necessary at any particular time and place in order that the disease should become general, but the practical value of this law is diminished by the fact that there is no means of recognizing the epidemic constitution except by the actual production of an epidemic. Plague, like all similar diseases, and in a specially high degree, is subject to the law of periodicity. Even when it is most strictly endemic it seldoms prevails continuously, but appears in definite outbreaks, or epidemics, with inter vals in which there are either no cases of plague or only so-called sporadic cases. This may be partly due to the general law that the susceptibility of the population to a special disease is exhausted by an epidemic, partly to the immensely increased transmissibility of the disease caused by the increased number of cases, so that when once a certain stage of severity has been reached the disease pro gresses in a far more rapid ratio. In most epidemics of plague there is at one time a sudden and alarming increase 1 Prus, Rapport sur la Peste, Paris, 1816, p. 133 ; Report of Com mittee of House of Commons, 1819, 101. 163 in mortality ; but, by a law not yet understood, each epidemic is liable to a spontaneous decline, which is some times sudden. This may be connected with rise or fall in the temperature of the air, but is not always so. The disease may be dormant during the cold or hot weather (as the case may be) and reappear when the temperature is favourable again, but not necessarily. It is generally agreed that plague is transmissible to another country only when it is epidemic, and not from sporadic cases. Incubation. It is a very important question what time may elapse between a person receiving the poison and showing symptoms of the disease. The usual time of incubation appears to be from three to five days. In certain very malignant epidemics this period may be shortened, and, it is thought, reduced to even less than a day. In rare cases incubation may be prolonged to eight days. There are doubtful accounts of ten days incubation. Generally a week s observation - would show whether a suspected person was really affected or no. It has been thought that articles contaminated by contact with plague patients may retain the power of communicating the disease for weeks, months, or even years ; but of this there is no adequate proof. Treatment. No special line of treatment has proved efficacious in checking the disease once established. Special symptoms are treated in accordance with the ordinary rules of practice, and need not here be con sidered. Free ventilation appears to be of the greatest service in preventing the spread of the disease, and pro bably in promoting recovery. Prevention. There can be no doubt whatever of the efficacy of hygienic measures in rendering a locality unsuitable for the spread of plague. Such measures include, not only personal cleanliness, but especially the removal of all foul organic matters, good drainage, and prevention of overcrowding ; all such measures might be looked upon by our readers generally as matters of course, but are quite unknown in most of the homes of plague. Since there is no doubt that plague may be carried from places where it prevails epidemically, measures to prevent such importation cannot be neglected. The best known of such measures is the system of quarantine first produced about 1480. See QUARANTINE. The efficiency of quaran tine has been much discussed, and very strong opinions have been expressed for and against it. The subject is too large for discussion here ; but it would appear that, while the system as originally applied in the Mediterranean, when traffic was comparatively slow and infrequent, and when European cities presented an extremely favourable soil for plague if introduced, was a real protection, the regulations have long ceased to correspond to the actual state of medical knowledge ; and, in addition, it would be impossible to apply them to our crowded traffic. The alternative is a system of medical inspection of all arrivals in our ports, and strict isolation of ships in which plague has occurred or is suspected. Such a ship should then be treated as an infected house. Prevention of the Spread of Plague. When cases of plague have once occurred in a town or on board a ship in port, the house or ship should be emptied of its inhabit ants, the sick removed to a hospital, the sound placed in an isolated building and subjected to observation for at least a week, or, better, ten days. The clothes of sick persons had better be burnt, their bedding and furniture completely disinfected. The house should in the mean time be rigidly closed until it has been disinfected. If these measures are taken in time, there can be no objection to allowing free emigration of the population. Isolation of the place by a " sanitary cordon " would only be possible
- Prus, Rapport, p. 196.