Parasitism and Immunity. 57 



This unequal infection is the crux of the whole position, and if we could 

 account for it satisfactorily it would help us to understand the conditions 

 under which infection occurs. At the present time the germ theory of 

 infectious disease in the human subject is on the eve of being considerably 

 modified, and this has a direct bearing on infection in plants. The sjnut 

 fungus produces its spores, and these, like the tubercle bacilli for instance, 

 give rise to certain symptoms in the host, which only occur in association 

 with this particular kind of organism. Further, the spores, like the patho- 

 genic or disease-jDroducing germs, have been isolated from the plant-bodv, 

 cultivated on artificial nutritive media, and then studied in the laboratory. 

 But it has been found that organisms which cannot be distinguished from 

 pathogenic germs exist in the tissues of persons who are apparently in perfect 

 health, and such persons have been designated as " disease -carriers." 

 Here is a case where the organism, supposed to be the definite causal agent, 

 is present without the symptoms, and the idea is gaining ground that the 

 so-called pathogenic germs are ordinarily harmless, but they require certain 

 conditions to develop their virulent properties. Just as the infection thread 

 from the germ of the smut can penetrate the plant but cannot produce 

 infection in the presence of substances known as chemotactic, so in the 

 human subject something is required to co-operate with or oppose the germs, 

 before they can be expected to show pathogenicity or the reverse. In the 

 plant, sugar is the great attraction for the parasite, and since sugar is usually 

 associated with ferments, it may be that in both plant and animal something 

 of the nature of a ferment must be present to excite the germs to activity, 

 and so the " Ferment " theory of infectious disease may yet explain why 

 under seemingly similar conditions the germ is pathogenic or disease-producing 

 i n the one case but not in the other. 



