204 BACTERIA PATHOGENIC TO MAN 



sistent phylogenic characteristics which reappear when the organism 

 is placed in a previous environment. 



Transmission of Diphtheria. The possibility of the transmission of 

 diphtheria from animals to man cannot be disputed ; we have met with 

 one instance where a cat had malignant diphtheria, and many other 

 animals can be infected, but there are no authentic cases of such trans- 

 mission on record. So-called diphtheritic disease in animals and birds 

 is usually due to other micro-organisms than the diphtheria bacilli. 



Let us consider some of the means by which the disease may be com- 

 municated. In actual experiment the bacilli have been observed to 

 remain virulent in bits of dried membrane for twenty weeks. Dried 

 on silk threads Abel reports that they may sometimes live one hundred 

 and seventy-two days, and upon a child's plaything which had been 

 kept in a dark place they lived for five months. The virulent bacilli 

 have been found on soiled bedding or clothing of a diphtheria patient, 

 or drinking-cups, candy, shoes, hair, slate-pencils, etc. Besides these 

 sources of infection by which the disease may be indirectly transmitted, 

 virulent bacilli may be directly received from the pseudomembrane, 

 exudate, or discharges of diphtheria patients; from the secretions of the 

 nose and throat of convalescent cases of diphtheria in which the virulent 

 bacilli persists; and from the healthy throats of individuals who acquired 

 the bacilli from being in contact with others having virulent germs on 

 their persons or clothing. In such cases the bacilli may sometimes 

 live and develop for days or weeks in the throat without causing any 

 lesion. When we consider that it is only the severe types of diphtheria 

 that remain isolated during their actual illness, the wonder is not that 

 so many, but that so few, persons contract the disease. It indicates 

 that very frequently virulent bacilli are received into the mouth, and 

 then either find no condition there suitable for their growth or are 

 swept away by food or drink before they could effect a lodgement. 



Susceptibility to and Immunity against Diphtheria. An individual 

 susceptibility, both general and local, to diphtheria, as in all infectious 

 diseases, is necessary to contract the disease. Age has long been recog- 

 nized to be an important factor in diphtheria. Children within the 

 first six months of life are but little susceptible, the greatest degree of 

 susceptibility being between the third and the tenth year, while adults 

 are almost immune. 



As the result of animal experiments, it is now known that an artificial 

 immunity against diphtheria can be produced, at least for a consider- 

 able length of time, by the development of substances directly antidotal 

 to the diphtheria toxin. By the inoculation of virulent or somewhat 

 attenuated cultures or of diphtheria toxin, Fraenkel, Behring, Wernicke, 

 Aronson, Roux, and since then many others, have succeeded in immun- 

 izing animals; but the most important and valuable results are those 

 which have been obtained by Behring, in conjunction with others, 

 who showed that the blood of immune animals contains a substance 

 which neutralizes the diphtheria toxin. The blood serum of persons 

 who have recovered from diphtheria has been found also to possess 



