PROBLEMS OF PATHOGENIC MICRO-ORGANISMS 237 



depends on some original offensive power of the microbe which it 

 accidentally possessed, such as toxin production, or the presence of 

 intracellular toxins combined with defensive powers. These, pos- 

 sessed independently of the host, were probably the entering wedges 

 to be further developed or dropped, according to necessity. It is more 

 than probable that all species of bacteria which possess these rudi- 

 mentary invasive powers have already availed themselves of the 

 opportunity to become parasites of animal life on the one hand, of 

 vegetable life on the other, and that no startlingly new diseases will 

 arise from saprophytic forms. 



Subsidiary problems there are, however, concerning the modifica- 

 tions and readaptations of the parasitic forms already in existence. 

 These may be grouped under two heads: 



(1) The transfer and adaptation of parasites from one host species 

 to another. 



(2) The increase of invasive properties of parasites of the same 

 host. 



Are there any new diseases likely to appear as a result of the suc- 

 cessful adaptation of parasites of higher animals to the human sub- 

 ject? This is a legitimate question, though difficult to discuss, for 

 want of material at present. Among the more important possibilities 

 I will simply mention the bovine tubercle bacillus and the hog-cholera 

 group of bacteria. The larger number of parasites on animals are so 

 specialized, however, their receptor apparatus, according to Ehrlich, 

 may have been so curtailed, that parasitism on a relatively distant 

 species may be impossible. 



As regards the second problem, that microbes may gain in invasive 

 power on the same host, the principle I have endeavored to establish 

 would stand in the way of any rise in virulence, because the most 

 invasive forms of a varying species would have the least chance for 

 transmission. Whatever increase in disease-producing power may 

 be acquired must be gained under special conditions, one of which is 

 association with other microbes. Thus, if we could conceive of the 

 same streptococcus, originally an inhabitant of the normal throat, as 

 passing on account of some series of accidents through the bodies of 

 a number of scarlatina patients, this streptococcus might thereby rise 

 temporarily to the level of a serious menace to the throats and perhaps 

 other organs of relatively healthy people. 



Again, certain microbes, like B. coli, the pneumococcus and men- 

 ingococcus, may, by living upon catarrhal mucous membranes, and 

 passed from case to case, acquire enough temporary pathogenic power 

 to cause localized epidemics under favorable conditions. Any advan- 

 tage thus gained would soon be sacrificed, and the microbe return to 

 the normal condition, unless a satisfactory mechanism of transmission 

 could be established. 



