PSEUDODIPHTHERIA. 155 



For the recognition of the non-toxin-producing form, ex- 

 periment on animals is the only means of differentiating. 



What appear to be true diphtheria bacilli have been found 

 in the throat and mouth in about 1 per cent, of a number 

 of healthy persons examined, but generally in individuals 

 who have come into contact with diphtheria patients, or when 

 diphtheria was prevalent in the community at the time of 

 the examination. Those persons are always a source of 

 danger to others, and they no doubt are in a great measure 

 responsible for the spread of the disease. 



The experiments of Roux and Yersin have shown that the 

 various cultures of diphtheria bacilli have different potency 

 in the production of toxins, and that occasionally bacilli 

 grown under conditions, the same as much as possible, may at 

 different times produce more or less toxins, and of a greater 

 or lesser virulence. These facts bacteriologists are in no 

 position to explain, and the toxicity of a diphtheria culture 

 may only be determined by experimentation on animals. 



The Antitoxin Treatment of Diphtheria. 



The discovery made by Roux, that the diphtheria bacilli 

 secrete a toxin which, when injected into susceptible ani- 

 mals, produces all the symptoms of true diphtheria, was 

 soon followed by the discovery of Behring, which showed that 

 the blood-serum of animals injected with the bacilli of diph- 

 theria contains a substance which when inoculated into sus- 

 ceptible animals is able to immunize them from lethal doses 

 of the bacilli. 



, These substances, called antitoxins, are obtained from ani- 

 mals having little or no susceptibility to the disease, and 

 they have been used extensively both in the prevention and 

 cure of diphtheria since 1894. 



These antitoxins as exhibited therapeutically are obtained 

 from the blood-serum of horses, as first suggested by Roux, 

 and are prepared as follows : 



Immunization. A good-sized horse, which has been demon- 

 strated to be free from tuberculosis and glanders, by the injecting 



