CHAPTER VI. 



THE EFFECTS OF PAEASITES ON" THEIE HOSTS. 



Parasitic Diseases. 



Feom what has already been said of the life-history of parasites, and 

 especially of the Entozoa, it is evident that they influence in a most 

 important way the health and even the life of their hosts. But the 

 existence and amount of this influence was firmly established only by 

 the discoveries of recent decades. From this time a rational theory 

 of parasitic diseases, and a true insight into the deep significance of 

 this important branch of medical science, must date. Not that the 

 idea of parasitic diseases was something absolutely new; on the 

 contrary, from the earliest times men knew and feared the injurious 

 effects of these unbidden guests, and feared them perhaps even more 

 than they knew them. 



In order to form a correct estimate of the pathological significance 

 of parasites, it is necessary to cast a glance at the literature upon the 

 question of the seventeenth and eighteenth centuries.^ 



There was then no grievous and dangerous malady which parasites, 

 and especially intestinal worms, were not thought capable of exciting. 

 Dysentery, scurvy, hydrophobia, and even the dangerous epidemics of 

 the Middle Ages, such as plague and small-pox, were all described as 

 parasitic diseases. With each disease they associated a particular 

 parasite, just as we now sometimes speak of the cholera-Bacillus, and 

 other similar creatures, as the transmitters of certain specific diseases. 

 They supposed, further, that these originators of disease lived either 

 in the alimentary canal, or under the skin, or in the blood, and thence, 

 according to their nature, infected the whole organism in diverse ways. 

 Nor was this opinion held by individuals only, but by many, and 

 partially even by the most famous representatives of the pathology 

 of the time (Leeuwenhoek, Hartsoeker, Andry, and others). 



The possibility of such extravagant opinions is now the subject of 

 incredulous wonder. To understand them it is necessary to realise 

 the condition of medical science at that time. On the one side there 

 was inaccuracy of diagnosis, £Cnd almost entire ignorance of pathological 



' I specially recommend Andry, " Traits sur la gdn6ration des vers dans le corps de 

 rhoirime," Paris, 1700, of which a new edition and German translation have since appeared. 



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