340 TYPHOID FEVER. 



sufficiently explains the failures in the later stages. The second 

 and great difficulty in the way of accepting the etiological 

 relationship of the B. typhosus lies in the comparative failure to 

 cause the disease in animals. We have noted, however, that in 

 nature animals do not suffer from typhoid fever. 



3. The observations of Pfeiffer and others on the protective 

 power against typhoid bacilli shown, on testing in animals, to 

 belong to the serum of typhoid patients and convalescents, and 

 the peculiar action of such serum in immobilising and causing 

 clumping of the bacilli (vide infra) are also of great importance. 

 Additional important evidence of the typhoid bacillus being the 

 cause of typhoid fever is found in the fact that vaccination by 

 means of the dead bacilli (vide infra) has a marked effect in 

 preventing the disease arising in a protected population exposed 

 to infection, and also in lowering the mortality when the fever 

 attacks those who have been inoculated. These facts may thus 

 be accepted as indirect but practically conclusive evidence of the 

 pathogenic relationships of the typhoid bacillus to the disease. 



According to our present results we must thus hold that the 

 bacillus typhosus constitutes a distinct species of bacterium, and 

 that there is every reason for accepting it as the cause of typhoid 

 fever. Evidence of an important nature confirmatory of this 

 view is, we think, found in the fact that cases have occurred 

 where bacteriologists have accidentally infected themselves by 

 the mouth with pure cultures of the typhoid bacillus, and after 

 the usual incubation period have developed typhoid fever. 

 Several cases of this kind have been brought to our notice and 

 are not, we think, vitiated by the fact that other similar instances 

 have occurred without the subsequent development of illness. 

 These latter would be accounted for by a low degree of suscepti- 

 bility on the part of the individual or to a want of pathogenicity 

 in the cultures. 



The Serum Diagnosis of Typhoid Fever. This method of 

 diagnosis is based on the fact that living and actively motile 

 typhoid bacilli, if placed in the diluted serum of a patient suffer- 

 ing from typhoid fever, within a very short time lose their 

 motility and become aggregated into clumps. The researches 

 which led up to the discovery will be described in the chapter 

 on Immunity. We shall find that in many diseases the serum 

 has this property of causing agglutination of cultures of the 



