CYTORYCTE8 VARIOLA. 735 



tact with patients, and the disease may be carried 

 to others from the sickroom by a healthy person. 

 Epidemiologic experience teaches that the virus is 

 one of great resistance and tenacity. 



The incubation period in variola falls within the Cyclic 



. , , , Nature of 



extremes of eight to twenty days, most commonly symptoms. 

 from nine to fifteen days. The stage of invasion, 

 or the primary fever, terminates the incubation 

 period, and during this time the initial rash ap- 

 pears, accompanied by moderate hyperleucocytosis. 

 On the third to the fourth days the remission sets 

 in, the number of leucocytes in the blood decreases 

 to normal or below normal, and cutaneous lesions 

 make their appearance, and in the course of forty- 

 eight hours show a vesicular nature. When the 

 umbilicated vesicles are changed into pustules the 

 temperature again rises (secondary fever) and hy- 

 perleucocytosis again develops. This much only 

 of the clinical picture is mentioned to emphasize 

 the cyclic nature of the phenomena ; one may well 

 suspect that the organism causing such a disease 

 undergoes particular phases of development which 

 in some way are related to the well-known clinical 

 cycle. 



Epidemics are sometimes of so mild a charac- variations 



XT i j/i i- i j T i T i T in Virulence. 



ter that the patients are not bed-ridden and may 

 be found in the pursuit of their occupations in 

 spite of well-marked eruptions. Such occurrences 

 can be referred only to a virus of low pathogeni- 

 city. Even mild epidemics, however, may be ac- 

 companied by severe and fatal cases. Cases of am- 

 bulatory smallpox are most important factors in 

 spreading the disease. 



We have nothing more than presumptive knowl- 

 edge concerning the distribution of the virus in 



