22 INTRODUCTION 



anthrax and the swine plague bacillus, without any concomitant 

 evidence of disturbance of the host. There may thus be infection 

 of a high order without any evidence of associated disease. 



Evidently, then, it is necessary to distinguish sharply between 

 (a) mere surface invasions, (6) infection proper, and (c) infectious 

 disease. The first subject belongs essentially to the epidemiologist 

 and the sanitarian, while the study of infection and infectious dis- 

 ease engage the attention of the immunologist. 



