532 INFECTION AND IMMUNITY. 



A number of different principles have been fol- 

 lowed in immunizing with cultures. It seems that 

 virulent strains cause a higher degree of immunity 

 and a serum of higher protective power for other 

 animals than strains of low virulence. On this 

 account Marmorek, and also Aronson, immunize 

 horses with streptococci, the virulence of which has 

 been pushed to a very high point by passing them 

 through rabbits. Strong resistance is induced by 

 this method, and the immune serum, particularly 

 that of Aronson, shows distinct protective power 

 for other animals. Such serums, however, have 

 the highest protective power against the particu- 

 lar strain which was used for immunization, al- 

 though the serum of Aronson is not devoid of pro- 

 tective powers against other pathogenic strains. 

 Concerning the serum of Marmorek there are di- 

 vergent opinions. In the hands of Marmorek it 

 is highly protective in animal experiments; others 

 have found it without value. The method of Mar- 

 unity or MUI- morek and of Aronson rests not only on the basis 

 * that strains of the highest virulence will give the 

 strongest serums, but also on the assumption of 

 the unity of all pathogenic streptococci. If all 

 are alike in their biologic and pathogenic proper- 

 ties, a serum which protects against one should 

 protect against all. As pointed out, there is at 

 present not sufficient ground for considering the 

 streptococci of erysipelas, scarlet fever, rheuma- 

 tism, sepsis, etc., as independent species. By cul- 

 tivation and passage it is possible to so modify any 

 one of them that it is indistinguishable from the 

 others, on the basis of morphology and patho- 

 genicity. On the other hand they are not all 

 identical in some very important properties. For 



