EPIDEMIOLOGY OF INFLUENZA 433 



AVhat is at the bottom of this succession of waves, it is hard 

 to say. The most natural explanation would be that there is a 

 short lived immunity, conferred by the original attack, and that the 

 second and third waves appear at times when the infectious agent 

 is still widely distributed while the immunity has waned. It is 

 very difficult to get at the facts, but serious attempts are being 

 made, particularly by Frost and others. Studies by Jordan and 

 Sharp 22 at the Great Lakes Training Station indicate that no marked 

 immunity existed twelve to fifteen months after the first attack. 

 This, too, seems to be the conclusion reached by Frost who states 

 that in Baltimore those persons who were attacked during the 1918 

 to 1919 epidemic, showed no relative immunity during the epidemic 

 of 1920. It would seem in general that an almost universal infection 

 of a community with the first mild disease conferred a short lived 

 immunity. As a consequence of this, the epidemic would burn itself 

 out and wane. Gradual return of susceptibility in the course of the 

 subsequent period of months, not, however, bringing the community 

 back to the very low universal resistance which it was characterized 

 by before, would now create a soil in which reintroduction of the 

 infectious agent could produce many cases, but in which spread 

 would be less rapid and extensive. Such a view, however, must 

 be regarded as purely tentative, and it is hoped that a final study 

 of the statistics gathered during the great war epidemic will clear 

 this matter up. 



It is a difficult question to decide where the last war epidemic 

 began. After the 1889 epidemic, it seems that the disease may have 

 remained endemic in a great many different parts of the world. It 

 may have been so universally distributed that we cannot speak, 

 in this case, of a definite focus in China or Turkestan, as this was 

 done in past epidemics. Before 1889, the world was not so con- 

 tinuously traveled over by large numbers of people. There was 

 less travel by railroad communication, steam-ship lines, etc. It may 

 be that this development of civilization has completely altered the 

 epidemic conditions prevailing in regard to influenza. It has been 

 suggested in the case of the last epidemic however, that it came 

 from the East, as did previous outbreaks. McNalty in an article in 

 Nelson's System states that the disease was prevalent in March, 

 1918, in China, and that in April cases appeared on a Japanese ship 

 in a Chinese port. Frost who has given particular attention to this 



^-Jordan and Sharp, Jour, of Infec., Dis., May, 1920, p. 463. 



