THE DIPHTHERIA BACILLUS 183 



The severity of the disease produced by the diphtheria bacillus 

 cannot be regarded as an index of the virulence of that particular 

 strain. Association with other organisms and the presence of 

 varying amounts of antitoxin in the blood of the patient may mask 

 the real power of the invader. Descendants of the same organism 

 may give rise to mild symptoms in one person and to a fatal infec- 

 tion in another. 



Persistence of Diphtheria Bacilli in the Throat. The length 

 of time the bacilli continue to live in the throat after apparent 

 recovery varies greatly. " Diphtheria bacilli disappear in about 

 50 per cent of cases by the time the local membrane has dis- 

 appeared. They persist in about 5 per cent of persons at the end 

 of two months, about 2 per cent at the end of three months, and 

 approximately 1 per cent continue as chronic bacillus carriers." 

 (Rosenau.) 



Immunity. The fact that fully toxic bacilli have been fre- 

 quently found in the throats of healthy persons who have been 

 brought in contact with diphtheria patients, yet who have not con- 

 tracted the disease, demonstrates that diphtheria, like other infec- 

 tious diseases, requires not only the presence of the specific organ- 

 ism but also a susceptibility on the part of the individual. It is 

 estimated that about 70 per cent of all persons are protected from 

 infection because of an antitoxin present in their blood. Condi- 

 tions therefore which impair vitality and diminish the production 

 of specific antibodies increase susceptibility. 



Immunity following an attack of diphtheria usually lasts for 

 several months or even years. Occasionally, however, it is of 

 much shorter duration. Infants and adults possess relatively 

 more immunity than young children between the ages of two and 

 ten years. It is known that young animals born of immunized 

 mothers inherit a certain degree of resistance. This may explain 

 the relative insusceptibility of children during the first months of 

 life. Passive immunity is only of short duration; the antitoxin 

 injected usually disappears from the blood in less than three weeks. 

 The nature of antitoxin and its prophylactic and therapeutic use 

 is discussed in Chapter II. 



