﻿314 STRONG. 



from fifteen to twenty-five successive passages the maximum virulence 

 of the spirillum can usually be reached if the attenuation has not been 

 too great. Again, if such a culture which has attained its maximum 

 virulence be left upon artificial media for a few weeks, the virulence 

 becomes greatly reduced. This process of increasing and of decreasing 

 the viridence of the cholera organism may be repeated at will. 



On the other hand, we have seen with the pest bacillus that it is 

 impossible to reproduce these phenomena. The pest bacillus does not 

 easily undergo autolysis in old agar cultures and it produces no ferments 

 which are capable of causing its own destruction. Virulent pest cultures, 

 whether they remain continuously in the original agar culture upon which 

 they were first inoculated or are transplanted from culture to culture, 

 remain alive and frequently retain their full virulence, although many 

 instances of spontaneous loss of this property occur. Strains of the 

 organism of moderate virulence, but which still kill guinea pigs, can not 

 be rendered perceptibly more virulent by repeated passages from animal 

 to animal, unless perhaps it be under certain special conditions and the 

 same applies to avirulent strains of the organism which rarely kill the 

 animal except in very large doses. 



This evidence is particularly interesting from an epidemiological 

 standpoint, when we compare epidemics of cholera with those of plague. 

 With the former disease, the epidemic — no matter which type it may 

 later assume — probably begins either by the introduction of the virulent 

 organism into a new district by a sick individual who has traveled 

 from a region where the disease is alread}' epidemic, or by a few cases 

 of infection in the neighborhood of the new district, which are usually 

 of a mild character and may entirely fail to attract attention. In case 

 the water suppfy of the region becomes infected from some of these cases, 

 the organism having increased in virulence in its passage through the more 

 susceptible individuals with which it has come in contact, the epidemic 

 may assume an explosive character which is very marked and thousands 

 of people may be stricken within a day or two. If the epidemic does 

 not extend through the water supply, but by direct or indirect contact 

 with cases of infection, a better opportunity for its study is usually 

 afforded. It may then frequently be observed, although obviously this is 

 not always the case, that as the epidemic extends the virulence of the 

 cases increases to a maximum and then as the number of cases slowly 

 subsides, the severity also usually becomes reduced, so that within a 

 short period of time, at most within a year or two, the epidemic has 

 either reached its maximum severity or has entirely disappeared. Such 

 outbreaks of cholera would appear partially to be due (leaving meteoro- 

 logical influences, relation to soil, dissemination of the organism, etc., 

 out of the question), to a rise of virulence of the cholera spirillum 

 which is brought about by the fact that the more or less attenuated 



