DRUG FASTNESS 249 



substances must be constructed, in order to produce the maximum 

 amount of effect with the least chance of leading to the development 

 of an insurmountable drug resistance. That this can be done, 

 Ehrlich himself has demonstrated in a perfectly satisfactory manner. 

 He could thus show that mice which had been infected with arsanil- 

 fast trypanosomes could be cured with arsenophenyl glycin, even at 

 a time when death seemed to be imminent. The problem will, of 

 course, be the more difficult the larger the number of drug-fast 

 strains that is possible, and not only this, but the larger the number 

 of serum-fast strains that may develop. For we must bear in mind 

 that the destruction of the parasites in question is of necessity 

 followed by the development of corresponding antibodies, which in 

 itself is, of course, a favorable occurrence. 



If, however, the destruction of the tryanosomes by the arsenical 

 preparation, for example, has not been complete, and if the resultant 

 antibodies do not succeed in killing off the rest, there is a strong 

 probability that a serum-fast strain will now develop and bring about 

 a relapse (relapse strain No. I). When some of these organisms 

 then die or are killed by a repetition of the dose of arsenic, if 

 indeed the strain is still susceptible to the same preparation, a new 

 type of antibody will be formed which will be specifically directed 

 against relapse strain No. I, from which a new serum-fast strain may 

 then develop, and cause a second relapse (relapse strain No. II), 

 and so on, the number of serum-fast strains being only limited by 

 the ability of the parasite to produce new receptors with which it 

 can attend to its nutrition. 



This implies, of course, that as the number of different kinds of 

 foodstuffs which the parasite can utilize, progressively diminishes, 

 a time will finally come when the infection will become eradicated 

 spontaneously. This might occur relatively early, so that the 

 infected animal or patient would actually receive the benefit of this 

 fractional destruction of the parasites, but, on the other hand, there 

 is a possibility that during each relapse vital structures may be 

 damaged to such an extent that the individual would not live long 

 enough to reach the point where the infection would at last have 

 exhausted itself. 



Examples in point are relapsing fever, on the one hand, and syphilis 

 and sleeping sickness, and possibly also malaria, on the other. In relap- 

 sing fever we thus have evidence that only three or four different serum- 



