70 IMMUNITY 



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for years against invasion and infection with the cytorcytes in viru- 

 lent form. A person who has variola cannot be vaccinated, sub- 

 sequently he is immunized against vaccinia by this attack of variola, 

 just as he can be immunized against variola by vaccinia infection. 



Since Jenner first discovered that cow-pox introduced into the 

 body prevented small-pox, it has been the world- wide custom to use 

 either the dried virus or liquid glycerinized virus from the cow or 

 human beings in the process of vaccination. It has been found that 

 human virus generally used was likely in rare instances to transmit 

 syphilis, so it is now the universal custom to use cow virus. This 

 virus is collected from fresh vesicles in calves or young heifers, as 

 clean as possible, as it is used as seed to inoculate the animals and 

 the operation is done under strict anti-septic precaution. After a 

 week the virus is collected under similar anti-septic precautions by 

 scraping the base of the vesicle with a sterile curette. The pulpy 

 substance thus obtained is mixed with glycerine and stored for a 

 month or more. The action of the glycerine is to rid the virus of 

 many of the bacteria, through, it is supposed, a hydrolytic action. 

 This virus is then rubbed into the skin of the individual to be vacci- 

 nated under strict aseptic precautions. At the end of a week, a 

 pearly white vesicle is formed, and it is then considered that vacci- 

 nation has "taken" and that the individual is protected against 

 variola. This action of immunization is supposed to be complete 

 on the fourth day after the virus has been introduced. This is a 

 matter that is difficult to decide, but the immunization process is, no 

 doubt, a very slow one, like every other immunizing process where 

 the immunity is autogenous and active, and not passive, as in the 

 case of diphtheria anti-toxin. 



There can be no doubt about this being one of the greatest boons 

 that mankind has ever received. Vaccination is attended with some 

 risk. Septic infection with streptococci sometimes follows, likewise 

 tetanus infection. In both instances this may be due to the contami- 

 nation of the vesicle on the calf before the virus is lifted, to pirty 

 methods, or to contamination after vaccinations, probably the latter. 



