REPORT ON THE LAWS OF CONTAGION. 91 



marked by regular intervals, and not traced to local causes. 

 Though the works of writers on epidemics give us no insight 

 into their causes, yet they contain excellent descriptions of the 

 phgenomena. Of these the following is a very general outline : 



1. Epidemic diseases do not observe any fixed cycles, nor can 

 we at all anticipate the periods of their return. Some epidemics, 

 however, are disposed to prevail most at particular seasons of the 

 year, as in spring and autumn. 



2. Epidemics seldom spread suddenly over very extensive re- 

 gions, but are observed to make a gradual, often a slow, progress 

 from one kingdom to another, from province to province, and 

 even from one locality to another not far remote. The influenza, 

 (a catarrh, accompanied with extreme debility,) which was epi- 

 demic in England in 1782, was noticed in the East Indies in 

 October and November 1781 ; at Moscow in December of the 

 same year; at St. Petersburgh in February 1782 ; in London it 

 was in full force in May; in France in June and July; and in 

 Italy in July and August. In the months of August and Sep- 

 tember it prevailed in Portugal and Spain*. The Asiatic cho- 

 lera, it is well known, made even a much more tardy progress 

 from the East westwards, and did not appear in England until 

 about fourteen years after it was known in British India. 



3. On the first appearance of epidemics, they are not always 

 distinguished by those symptoms which mark them in subse- 

 quent periods. The plague, for instance, for the few weeks after 

 its first invasion, is frequently unaccompanied by bubos or car- 

 buncles, which are seldom wanting when it has raged long in any 

 place. 



4. When diseases of this kind attack any country, they con- 

 tinue to spread until they have reached the period of their most 

 general prevalence, called their acme, and then decline. These 

 periods of commencement, acme, and decline, seldom coincide for 

 the same epidemic at different places. Of three localities, for in- 

 stance, not far remote from each other, the plague, which visited 

 England in 1666, was often observed at the same time to be first 

 showing itself in the one ; to be at its height in another ; and to 

 be on the wane in the third. The Asiatic cholera exhibited si- 



-milar irregularities in this and other countries. 



5. Epidemic diseases of the same name diifer materially, both 

 as to degree and to symptoms, at different visitations. The epi- 

 demic of one year maybe almost universally a mild and tractable 

 disease, and that of another extremely severe and dangerous. 



• See a general account of the Influenza, drawn up from the reports of me- 

 dical practitioners residing in various parts of England, in the Medical Com- 

 mioikalioiis, vol. i. 



