VACCINES IN PROPHYLAXIS OF DISEASE 221 



stage of incubation, the vaccine will not prevent 

 the development of the disease; but where the 

 child is not in the stage of incubation but about 

 to be exposed, the prophylactic vaccine will af- 

 ford immunity for at least two or three months 

 and probably for a longer time. 



SCARLET FEVER 



Prevalence The seasonal prevalence of scar- 

 let fever resembles that of diphtheria, increas- 

 ing in the fall of the year, due in part to the 

 gathering of children in the schools. There is 

 probably always more or less scarlet fever in 

 any thickly settled district in the temperate 

 zone. Scarlet fever varies greatly in intensity 

 in different epidemics. In some epidemics the 

 death rate, according to Rosenau, is 30 per cent ; 

 in others, it varies from 4 to 15 per cent. In 

 England and Germany the disease is always 

 endemic and f requently epidemic. In the north- 

 ern part of the United States, scarlet fever is 

 much more prevalent than in southern portions. 

 It has been noticed in the northern states that 

 once in every six years there is a general epi- 

 demic of this disease which frequently assumes 

 a malignant type. 



