394 THE ETIOLOGY OF TROPICAL DISEASES 



another. But the chief agency for spreading the disease to unin- 

 fected places consists of travellers. The lines of human communica- 

 tion are followed by the infection in a marked degree, especially lines 

 of steamship and railway. 



Plague is essentially a " filth disease," and it is frequently pre- 

 ceded by famine. Temperature and overcrowding exert an influence 

 upon it. The areas affected by the disease in the Middle Ages in the 

 seventeenth century, and in 1894-96, are alike in being characterised 

 by filth and overcrowding. There is little fear, speaking generally, 

 of the plague ever flourishing under Western civilisation, where the 

 conditions are such that even when it appears there is little to 

 encourage or favour its development* 



Administrative Considerations. Plague will not readily 

 fasten on any section of a population which is properly housed, 

 cleanly, and generally, in a sanitary sense, well-to-do ; rather it will 

 especially affect, if it obtains foothold in a district, insanitary areas 

 such as are peopled by the poorest class, and where overcrowding of 

 persons in houses and dirt and squalor of dwellings and of inhabi- 

 tants tend to prevail. 



In these circumstances, and from an administrative point of 

 view, the following facts respecting plague should be borne in 

 mind : 



(1) Plague has an incubation period of three to five (in excep- 

 tional cases of perhaps eight to ten) days. 



(2) Plague is wont, especially in its earlier manifestations, to 

 assume a mild form, or even to present anomalous symptoms, 

 tending to confound it with other and more innocent diseases. 



(3) Plague in all its forms must needs be regarded as personally 

 infective. 



(4) Plague affects rats as well as the human subject ; it may, 

 indeed, be found, causing mortality among these lower animals ante- 

 cedent to its definite invasion of the population. There can be no 

 doubt that the rat and man are, as regards plague, reciprocally 

 infective. 



Although Local Authorities should be on their guard against 

 plague, when cases occur at the ports or elsewhere in these islands, 

 it is not intended to suggest that there exists, under these circum- 

 stances, cause for alarm. There can be no doubt that, in this 

 country, hygienic conditions and methods of dealing with infectious 

 diseases are far in advance of those of former centuries wherein 

 plague was repeatedly epidemic in our populations; they are in 

 advance, too, of those in localities abroad, where plague has shown 

 itself formidable in recent years. During the past fifty years there 



* The most complete account of plague hitherto published is The Report of the 

 Indian Playue Commission, 1902, vols. i.-viii. (see in particular vol. v.). 



