24 INFECTION AND IMMUNITY. 



rent fever) ; third, there are a few diseases which 

 are peculiar to man and accordingly can not be re- 

 produced in experiment animals (leprosy, scarlet 

 fever, measles, etc. ) ; fourth, some infectious agents 

 are pathogenic for experiment animals, but do not 

 reproduce in them a clinical or anatomic condition 

 identical with that found in the original animal 

 (typhoid). 



Furthermore, failure to comply with all the re- 

 quirements enumerated does not, in some cases, 

 disqualify the organism as the causal factor. If 

 an organism is found constantly in characteristic 

 sites in a given disease and not in other infections, 

 and if at the same time other microbes are not 

 present or are present inconstantly or through ac- 

 cident, there could be little or no hesitation in ac- 

 cepting this organism as the cause of the disease, 

 even if it were impossible to cultivate it or to trans- 

 fer the disease to animals. The typhoid bacillus 

 has been cultivated from characteristic foci (stools, 

 blood, spleen, urine, rose spots) in such a large 

 number of cases, and the bactericidal and agglu- 

 tinating powers of the patient's serum against this 

 organism are so distinctive, that compliance with 

 the third law, though desirable, is not now essen- 

 tial. The conditions are similar in reference to 

 cholera and the cholera vibrio. 



The conditions are so unique in some diseases 

 that, although all Koch's laws have not been met 

 literally, certain equivalents have been met. To 

 illustrate, we may consider an anopheles mosquito 

 which has become infected with the plasmodium 

 of malaria by biting a malarial patient, as a cul- 

 ture medium ; and the transferring of the infection 



