228 HEALTH AND DISEASE 



are now constantly employed for the purpose of producing toxic fluids, 

 which are used for purposes of diagnosis and even of curing disease, as 

 illustrated by the present use of tuberculin for the detection of tuber- 

 culosis, mallen for the discovery of glanders in horses, a preparation of the 

 bacillus of diphtheria, modified by passing through the system of the horse, 

 for the cure of diphtheria in man, and the use of the modified cultivation 

 of the bacillus of tetanus for the cure and prevention of that disease both 

 in the higher and lower animals. 



The action of microbes in the production of contagious diseases was 

 demonstrated most perfectly by Pasteur in his researches on chicken 

 cholera and splenic fever (anthrax). In the course of these experiments 

 he proved not only that pure cultivations of the microbe outside the body 

 would produce the disease with absolute certainty in healthy fowls, but 

 also the still more important fact that by modifications in the method of 

 growing the organism the activity of the poison would become diminished 

 until it finally ceased. This discovery has been taken advantage of largely 

 by bacteriologists, with the result of ascertaining that an attenuated or 

 weaker virus may be obtained not only by modifying the method of cul- 

 tivation in certain media, but by passing the virus through the system 

 of an animal belonging to a different class from the one originally attacked. 

 Thus the bacillus of anthrax, after being passed through the guinea-pig, 

 loses its fatal activity on cattle (Sanderson and Duguid); the bacillus 

 anthracis of whatever source, after having been passed through the white 

 mouse, loses its fatal activity on sheep (Klein and Roy); and the same 

 organism, when passed through the South American rodent biscachia, 

 loses its fatal activity on cattle (Roy). That this weakening or loss 

 of virulence does not depend upon the death of the bacilli is proved 

 by the fact that if they are again cultivated in the ordinary way in 

 nutritive media they recover all their former quality — power to injure 

 and to kill. 



It has been asserted that there are at least three micro-organisms 

 which are without any pathogenic property, and which may, when grown 

 under certain conditions, acquire such properties. Thus the common hay 

 bacillus (bacillus subtilis), according to Buchner, may by cultivation be 

 transformed into bacillus anthracis; a common bacillus which is present 

 in the atmosphere may assume distinct pathogenic properties if grown in 

 an infusion of the seeds of abrus precatorius, and the common mould 

 aspergillus, when grown on alkaline material, assumes poisonous pro- 

 perties according to Grawitz. That these statements are absolutely with- 

 out foundation may be positively asserted on the evidence of numerous 

 experiments; and it may be affirmed, on the contrary, that in no case 



