650 INFECTION AND IMMUNITY. 



The fourth postulate of Koch, that of cultiva- 

 Cause of tion in pure culture and reproduction of the dis- 



eage ky means of such pure cultures, has not yet 

 been carried out. The occurrence of the organism 

 as described has, however, been such strong evi- 

 dence that the Spirochceta pallida is accepted as 

 the cause of syphilis. 



infection. Infection usually is venereal. It is not defi- 

 nitely known whether a defect of the surface of the 

 prepuce, glans, vagina, etc., is essential for infec- 

 tion. The epithelium in these localities is so deli- 

 cate that defects of microscopic dimensions may be 

 easily produced, and infection may take place 

 through such defects as through grosser lesions. It 

 is well known that the lip, tongue, conjunctiva and 

 finger may be the seats of primary lesions, and it 

 is probable that no part of the body surface is 

 immune when the virus is introduced suitably. 

 virulence. Clinical experience indicates that the virulence 

 of the SpirocJiceta pallida is not uniform. It is 

 possible that certain strains are more likely to 

 bring about "post-syphilitic" diseases than others. 

 That the resistance of the organism outside the 

 body is low seems evident from the fact that trans- 

 mission is practically unknown except as it occurs 

 by direct contact. Neisser destroyed it by heating 

 to 60 C. for thirty minutes, but at this tempera- 

 ture for ten to twenty minutes its virulence for 

 monkeys was retained. 



Prophylaxis demands no principles not generally 

 known. 



Susceptibility to syphilis varies a great deal, not 

 in the sense that some are immune, but in that a 

 more virulent type of disease develops in some than 

 in others. This is a condition, however, which 



