104 BACTERIOLOGY FOR NURSES 



ant, and a little hardier still are the typhoid, diphtheria, and 

 tubercle bacilli and the staphylococci. 



The " carrier " is perhaps one of the most potent factors in 

 the spread of disease and one of the most difficult to control since 

 he can only be detected by laboratory examination, and even 

 when detected cannot always be isolated. Fortunately in many 

 cases quarantine is not necessary. Specific treatment and clean- 

 liness may speedily cure the condition or render him less dan- 

 gerous. 



Koch's Postulates. According to Koch an organism can be 

 considered the causal agent of a given disease only after it has 

 fulfilled certain requirements: (1) it must always be associated 

 with the disease; (2) be isolated in pure culture; (3) produce 

 the disease when inoculated into a healthy animal, and (4) be 

 obtained again in pure culture. For a long time these conditions 

 were accepted as the only proof of such a causal relationship. 

 Recent studies in immunology and the demonstration of specific 

 serum reactions has, however, rendered such a procedure for the 

 most part unnecessary.' 



