DIPHTHERIA 197 



although during this time the virulence is gradually 

 attenuated. If, however, at any time previous to its 

 ultimate disappearance the organism be transplanted into 

 a suitable culture medium, it can re-acquire its full initial 

 virulence. Such a medium may be a human throat in 

 a suitable condition, and the research therefore establishes 

 the fact that drinking water must be added to the list of 

 the means capable of spreading diphtheria. Its importance 

 in this respect is the greater in the light of the entirely 

 independent results of Keyes, showing the great capacity 

 of the organism to survive in damp earth, which, when 

 once infected, may for very long periods continue to infect 

 any water flowing through it. 



Faulty sanitary conditions may also assist in the spread 

 of this disease by preparing the throat for the bacillus, and 

 may in this way apparently give rise to cases which would 

 never have arisen had it not been for the existence of such 

 conditions. 



It is also a matter of common experience that an epidemic 

 of true diphtheria is sometimes preceded by a prevalence 

 of 'sore throat,' which seems to gather in intensity till 

 cases arise of which the clinical character shows them to 

 be undoubtedly true diphtheria. No doubt the systematic 

 bacteriological examination which is now being undertaken 

 in several districts will do much to increase our knowledge 

 of these obscure points. 



It has been found that during diphtheria epidemics 

 dogs, cats and cows may all suffer from a disease which 

 appears to be identical with human diphtheria. 



Pathogenesis. — The incubation period varies from two to 

 seven days, but is usually from about two to four days, 

 while the mortality due to diphtheria is about 0'20 per cent, 

 of the total death-rate. 



In a typical case of diphtheria a white membranous 



