150 CONTEMPORARY SCIENCE 



how they reach their maximum or peak, and then how 

 they fall away again. Indeed, we now construct easily 

 and recognize readily the epidemic curves of different 

 epidemic diseases. But it is to be hoped that a new era 

 is appearing in the study of epidemiology in which experi- 

 ment may play a part along with observation, statistical 

 and other. Already beginnings are being made in the at- 

 tempt to define the distinction between the potentially 

 fluctuating grades or power of infectivity and degree of 

 virulence, taking the former to mean the natural propen- 

 sity which a microbe displays in penetrating the ordinary 

 portals leading into the body and its ability to survive and 

 multiply there, and the latter the capacity to overcome the 

 natural defenses when artificially inoculated. This is a 

 field clearly approachable by experiment, using small 

 laboratory animals, among which arise from time to time, 

 and much as happens with man himself, destructive epi- 

 demics induced by known microbes. Finally, there is the 

 field in which not a single species of microbe is concerned 

 but more than one, the first preparing, the other utilizing 

 the prepared way for its more vicious purposes. Fre- 

 quent examples of the last condition are observed among 

 the lower animals, in which, of course, the opportunities 

 for study are superior to those existing in man ; but re- 

 cent experiences in this and other countries during the 

 influenza epidemic carry conviction of this relationship, 

 since the original disease is recognized to be not of severe 

 nature, while the pneumonia engrafted upon it is admit- 

 tedly of highly fatal character. 



My purpose in reviewing some of the notable events 

 and tendencies in bacteriology which have come to light 

 in the past twenty-five years has been to present to your 

 consideration the achievements in one branch of modern 

 medicine, and to indicate the relation subsisting between 

 medicine and the more fundamental sciences of physics, 

 chemistry and biology. Bacteriology has depended also 



