742 INFECTION AND IMMUNITY. 



tion period of vaccinia is about three days less 

 than that of smallpox, successful vaccination pro- 

 tects within a limited period following exposure. 

 Immediate vaccination is demanded in case of 

 exposure. Healthy infants may be vaccinated 

 within the first six weeks or two months of life 

 and at any earlier period in case of exposure, 

 immunity An attack of smallpox confers prolonged and, 



nl Suscep- i _/ 



with lew exceptions, lasting immunity. Second 

 and even third attacks have been described. It is 

 known that those who have had smallpox may be- 

 come susceptible to vaccination after a period of 

 time. Susceptibility varies a good deal with age. 

 During the ages of from two to fourteen years 

 the disease is less common than between fifteen 

 and forty, and after this period it again decreases 

 in frequency. Undoubted instances of natural 

 immunity to smallpox occur, but they are very 

 rare. 



Leucocyte*. Smallpox is accompanied by a leucocytosis 

 which is peculiar because of the large number of 

 mononuclears. There is a slight rise in the 

 number of leucocytes during the first febrile on- 

 set, a fall to almost normal during the remission, 

 followed by a second rise, which may be as high 

 as 16,000 to 20,000. Fatal cases show a terminal 

 hypoleucocytosis (Magrath, Brinckerhoff and Ban- 

 croft). Large numbers of lymphocytes are also 

 found in the pustules (Eoger). Nothing of a sat- 

 isfactory nature is known concerning the relation- 

 ship of the leucocytes to recovery and immunity. 



There is no serum therapy for smallpox. The 

 interesting observation has been made, however, 

 that the serum of convalescents or of vaccinated 



