INFLUENZA. 367 



were twelve distinct epidemics, from which we may conclude that the intervals are 

 in reality much shorter. 



Occasionally the disease is limited to a comparatively small area, but more 

 frequently it invades a large portion of the earth's surface. In some instances so 

 great has been its prevalence that almost all parts of the world have been attacked. 

 Its onset is in many cases remarkably sudden ; thus in the year 1837 it seized upon 

 all parts of the metropolis within the space of a very few days. It has been 

 observed to occur also at the same time on land and on board different vessels which 

 have had no communication either with the shore or with each other. Often enough 

 it breaks out simultaneously in many different places, but sometimes its progress 

 from country to country is comparatively slow. Thus it has spread over the whole 

 of Europe in six weeks, but it may take six months to do so. In any particular 

 country its progress may also be slow ; thus between the invasion of London and of 

 provincial towns, or of Scotland, weeks, or even months, may elapse. 



A curious circumstance in the history of these epidemics is that they appear to 

 travel or migrate from place to place, and this they do in spite of adverse winds 

 and variations in temperature. In spreading over a large tract of country, influenza 

 lias been observed to follow a regular course, usually from north or north-east to 

 the south and west. It has been known to pass from Chinese Tartary to Russia, 

 Germany, Holland, England, Scotland, France, and then to Italy and the Mediter- 

 ranean, or to America, in rapid succession. In its course it appears to pass over- 

 seas, and has, as we have said, been known to attack ships in mid-ocean. 



When it enters a large town it usually remains there from six weeks to 

 two months, but sometimes its stay is more protracted, as at Paris in 1831, 

 where it was prevalent more or less for nine or ten months. Ultimately, how- 

 ever, it always disappears, and in the intervals of the attacks isolated or sporadic 

 cases never occur. Where it comes from originally no one can tell Some 

 people think it always exists at some one spot and spreads from there, whilst 

 others maintain that under favourable conditions, whatever those might be, it may 

 originate anywhere. Usually, each nation attributes to its neighbour from whom it 

 derived the disease the unenviable honour of having originated it. Thus, the Italians 

 have called it the German disease ; the Germans, the Russian pest ; the Russians, 

 the Chinese catarrh, and so on ; these names affording, as will be seen, some indica- 

 tion of its usual tract. 



In passing through a country it does not attack all parts of it ; most commonly 

 it spares the villages and small towns, but sometimes even large towns escape. It 

 is generally mel with in cities before appearing in the towns and villages around. 

 In large cities an outbreak is usually made up of a number of localised attacks, 

 certain streets or districts being more frequently affected than others. The number 

 of people seized during an epidemic is usually very great. In London, in 1847, it 

 has been calculated that at least 250,000 persons suffered, in Paris between one- 

 fourth and one-half of the population, and in Geneva about a third. 



Influenza prevails on every soil and geological formation, and there is no 

 evidence to show that it is in any way connected with volcanic disturbances, as was 

 at one time asserted. It is not, as far as we know, in any way influenced by 



