PARASITES A KEAL CAUSE OF DISEASE. 123 



germs find the conditions of their development fulfilled. Just in pro- 

 portion to the number of germs introduced, and to the adaptation of 

 the environment to their wants, will the number of parasites increase 

 in the individual case. 



We might perhaps suppose that the developmental conditions of 

 parasites involved a certain pathological state, and might also assume 

 that parasites could not be developed except in unhealthy organisms ; 

 but in the impossibility of all proof this would only be blind adher- 

 ence to a dogma. It is true that the same has been asserted in 

 regard to the spores of fungi, and even in regard to bark-beetles 

 (Bostrichus) and vine-insects {Phylloxera), but here also the assumption 

 of a previously existing pathological state seems unwarrantable, having 

 neither proof nor probability. 



What leads me most decidedly to this conclusion is the ease 

 with which even the healthiest individuals may be experimentally in- 

 fected with Entozoa. Of course, the experiment does not succeed 

 with every kind of parasite, but only, as was before explained, with 

 those which find suitable environment. Even then there may be a 

 few cases in which the expected result fails. But our former observa- 

 tions have prepared us for such experiences. For the development of 

 a parasite requires the presence not only of certain specific factors, but 

 also of many individual ones. It might even be granted that the health, 

 and especially the nature of the organism to be infected, are not without 

 effect on the imported brood (in the case above mentioned (p. 85), in 

 which, after three weeks, the heads of Tcenia cmnurus showed hardly 

 any traces of further change, the animal under investigation had been 

 used some time before for an experiment with Trichina), but we have 

 never found the slightest ground for believing that the development 

 of the invading Helminth is promoted or even conditioned by any un- 

 healthiness of the animal experimented upon.^ 



Meanwhile, it is safe to assume that wherever there is a real con- 

 nection between the unhealthiness of a host and the indwelling para- 

 sites, it is the latter who are the efficient causes. 



It is, however, not only on a priori grounds that we are warranted 

 in maintaining that parasites may cause even very dangerous diseases. 

 Experimental helminthology has securely established this position. I 



' Statistics, which alone can decide in this case, show, on the contrary, that certain ill- 

 nesses — e.g.j chronic, and especially intestinal catarrhs — tend to remove parasites from the 

 diseased organ, or even to prevent their occurrence. Thus Gribbohm { " Zur Statistik 

 mensohl. Entozoen," Kieler Inauguraldissert. , p. 8, 1877), in chronic intestinal catarrh, 

 mostly in consequence of phthisical processes, found in 05 bodies only 16 (24'6 per 

 cent.), and in chronic catarrh of the large intestine in 18 bodies only 3 (16'7 per cent.) 

 cases of the stomachic Nematodes {Ascaris, Oxyuris, Trichocephalus), which are other- 

 wise so very frequently P'^^sf%^(WS9/^^'''£fi'^/4&'^#^ '^®°*- "^ ''^^ cases examined). 



