EPIDEMIOLOGY OF INFLUENZA 491 



difficult. However, MacNeal 10 who has written up the progress of 

 the epidemic among American troops in France cites a number, of 

 cases where a true connection between ship communication and 

 the outbreak of influenza could be observed. Hospital outbreaks, 

 such as the one described by Foster and Cookson/ 7 and prison 

 outbreaks, such as the one described by Stanley 18 for the San Quen- 

 tin Prison, very definitely prove the paramount importance of con- 

 tact infection. Stanley shows definitely that the disease was brought 

 in by an infected prisoner, and that prisoners in contact with this 

 one and with other infected inmates contracted the disease, while 

 those who were isolated in other buildings or not in particularly 

 close contact with others, were spared. As to the suddenness with 

 which the disease attacks a great many people in a community, 

 the chief point which would make it seem that contact might be 

 entirely responsible, this can probably be explained by the extreme 

 infectiousness, the large percentage of susceptibles, and the fact 

 that people in the early stages of the milder forms of influenza are 

 up and about and are freely mingling with their fellows. An 

 epidemic is rarely recognized in a large community until two weeks 

 or longer after the first cases have appeared. Parsons in his 

 studies of the epidemics in England in 1889 and 1890, calls attention 

 to the fact that influenza is not more rapid in its spread and epidemic 

 onset than was smallpox in the days before vaccination. He also 

 has found evidence that shows that in localities where the outbreak 

 seemed particularly explosive, this could often be traced to meetings 

 of crowds at conventions or other organizations at times just preced- 

 ing the beginnings of the epidemics. The explosiveness of the out- 

 breaks during the last epidemic was evident, particularly, in military 

 communities. 



Influenza epidemics are always followed by secondary and tertiary 

 waves during which the disease after a definite lapse of time seems 

 to return often in a more dangerous form. This has been noted in 

 almost all carefully studied epidemics. 



After the epidemics of 1729 and 1730, secondary waves occurred 

 in 1732 and 1733. After the 1780 to 1781 epidemic, another series 

 of waves followed in 1782. The 1788 epidemic was followed by 



1G M<K>Xc ( ,l, Arch, of Inter. Mod., 2., 1919, 657. 



17 Foster and Cookson, Lancet, 2, 1918, 585. 



"Ktunley, U. S. Pub. Health Serv. Eep., No. 19, May 9th, 1919. 



