Infectious Agents. 55 



Even in ancient times the idea prevailed that devastating 

 epidemic diseases \vere caused by a Hving contagion (co)itagium 

 vk'uni sive aiiimafiiiii) . The nature of these conta^rions re- 

 maincd hidden, Iiowever, to the physicians of antiquity ; and the 

 impossibihty of determination led, especiahy in the middle 

 ages, to the wildest conjectures as to the nature and origin of 

 epidemics. They were attributed to evil spirits, deemed punish- 

 ments from on High, fancied the results of supermundane pow- 

 ers, of influences of the stars ; their origin was sought in con- 

 ditions of the weather, in magnetic and meteorological processes, 

 in putrid gases and in peculiarities of the soil ; and the hidden 

 factor was characterized as a coiistititfio cpidcuiica or pcstilens. 

 It is only about fifty years that our conceptions as to the real 

 nature of epidemics began to become clearer and an assured 

 foundation became established by precise observation — when, with 

 the aid of the microscope, it became possible to demonstrate the 

 existence of low vegetable organisms as foreign and invading 

 elements in the diseased body and to determine the role which 

 they play in the production of disease. In the last few decades 

 this phase of science, stimulated and reorganized by the luminous 

 work of Louis Pasteur and Robert Koch, improved by many 

 technical aids and demonstrative methods, has fully disclosed 

 the developmental history of many infectious diseases. Advances 

 of tremendous significance for the whole of medical science, dis- 

 coveries and experiences of the greatest consequence in the com- 

 bating and cure of diseases, have been attained in this line of 

 study. 



The demonstration of the relation which a given microbe 

 bears to a given disease has been especially facilitated by the 

 success of artificial cultivation of microorganisms outside the 

 body (in vitro, upon nutritive media) and of production at will 

 at any time thereafter of the infectious disease by inoculation. 

 Such experiments have been made not only in animals, but also 

 in human beings many times ; and anyone conversant with the 

 subject can convince himself b}- combined cultural and inocula- 

 tion experiments that certain microbes cause certain diseases. 

 The objection that the microorganisms are not the cause but 

 the accompaniment or product of the disease can easily be proved 

 worthless. Of course there are microbes in the body which have 

 nothing to do with disease, merely surface inhabitants of the 

 integument and mucous membranes, able to penetrate into the 



