10 INTRODUCTION 



In 1834 the contaguim vivum of itch, the itch mite (Sar copies 

 scabei), a fairly large mite to be sure, was rediscovered and its 

 relation to the disease made evident. In 1837, the same year 

 in which Cagniard-Latour and Schwann established the relation 

 of living yeast to alcoholic fermentation, Donne described 

 vibriones (bacteria) in syphilitic ulcers, and Audouin amplified 

 the discovery of Bassis that muscardine, a disease of the silk- 

 worm, was caused by a mold (Botrytis bassiana) which was trans- 

 mitted from the sick to the healthy worms by contact or by air 

 currents. These discoveries furnished a great impulse to further 

 investigation. 



Henle (1840) reviewed the evidence then at hand and con- 

 cluded in a very logical way that the causes of contagious dis- 

 eases were to be sought for among the minute living micro-organ- 

 isms. He recognized that no human disease had yet been shown 

 to be caused by a micro-organism and he formulated the re- 

 quirements to be fulfilled in order to prove such a relation, 

 namely, that the microbe must be constantly present in the 

 disease, must be isolated from the infectious material, and must 

 then alone be capable of producing the disease. 



During the next twenty years, the attempts to discover the 

 cause of an infectious disease and to satisfy the postulates of Henle 

 were successful in several diseases due to molds, Favus (Achorion 

 Schoenleinii) 1839, similar skin diseases known as trichophytosis 

 and pityriasis and especially thrush, shown to be caused by 

 Oidium albicans by Robin in 1847; but in all the more important 

 diseases only failure resulted. The reawakened interest in con- 

 tagium vivum therefore again gradually faded away. 



During this time Pollender and Davaine and Rayer (1850) 

 had discovered the minute rods in the blood of animals sick with 

 anthrax, and in 1863 Davaine had proved the almost constant 

 presence of these rods in the disease and the possibility of trans- 

 mission by inoculation from one animal to another. 



Pasteur from 1865 to 1868 investigated the fatal disease of 

 silk-worms known as pebrine, discovered the microsporidium 



