572 SCIENCE PROGRESS 



of December, though a few cases were observed at an earlier 

 period. The northern, southern, and eastern districts of the 

 MetropoUs were chiefly affected. In January the disease 

 moved to the west and also to the north, so that its maximum 

 intensity in Wales, Northumberland, and Scotland followed 

 that in London by about two weeks. 



How rapid was the progress of the wave may be gathered 

 from the following figures, which refer to deaths in the London 

 area. 



Week ending : 



Dec. 3. Dec. 10. Dec. 17. Dec. 24. Dec. 31. Jan. 7. Jan. 14. 



29 14 43 54 151 353 55i 



The last figure marked the height of the wave. During the 

 period under review the number of deaths from bronchitis and 

 broncho-pneumonia increased rapidly, but the cases of lobar- 

 pneumonia were comparatively few. 



The epidemic was not confined to this country. It ap- 

 peared also in Scandinavia, Germany, France, and Italy. It 

 was later reported in America. The symptoms in all these 

 areas appear to have been similar : headache, pain in the back 

 and legs, and congestion of the throat with some bronchial 

 catarrh and an irritating and very persistent cough. The 

 gastric form was frequent, and in some cases skin rashes of 

 various kinds were met with. As is usual, most of the deaths 

 were due to complications of one kind or another. In the 

 epidemic of 191 8-1 9 the young adult was the chief victim. 

 On this occasion, however, the victims were mostly very young 

 children and old persons. No satisfactory explanation of this 

 phenomenon has been offered. 



The Annual Report of the Medical Research Council for 

 1920-21 contains an account of an interesting experimental 

 study of epidemiology being carried out at Charing Cross 

 Hospital by Dr. Topley. He has introduced into a relatively 

 large population of mice a virus affecting these animals and 

 tending to assume epidemic strength. The conclusions already 

 reached are that : 



1. An epidemic among a population which is increasing at 

 a constant rate always progresses in a series of waves, indicating 

 some periodic fluctuation of the conditions upon which the 

 spread of infection depends. 



2. The survival of susceptible individuals coming among 

 the infected population has been found to vary widely in relation 

 to the phase of the epidemic during which they are first exposed 

 to the risk of infection. 



