INTRODUCTION g 



sumption and rabies as contagious. Fracastorius (1546) during 

 the period of the great epidemic of syphilis in Europe, published 

 a book containing the first comprehensive discussion of the theory 

 of contagion. He recognized contagion by contact, by fomites 

 and at a distance. Soiled material of all kinds was included 

 under fomites, as also those healthy individuals capable of trans- 

 mitting disease, a phenomenon already recognized. Transmis- 

 sion by insects and animals was also included under this head. 

 The transmission "per dis tans'- was considered due to emanations 

 from the patient diffusing to a distance through the atmosphere. 



Kircher in 1658 claimed to have seen the living Gontagium in 

 the body in the form of minute worms, and his observations were 

 widely recognized. The objects he saw were not accurately 

 ^described but it seems very certain that they were not bacteria. 

 Probably they were the normal cells of the tissues. 



The discovery of bacteria by van Leeuwenhoek (1683) was 

 not immediately recognized as of importance for the germ theory. 

 Leeuwenhoek himself considered it impossible for his " animalcula" 

 to penetrate into the blood because of the compactness of the 

 epithelial tissues. 



Almost a century^ later, Plenciz (1762) maintained that each 

 infectious disease must have its own specific cause. Reimarus 

 (1794) also expressed the same opinion and considered these living 

 organisms to be of the order of infusoria or perhaps still smaller 

 beings not yet visible with the microscope. These ideas were 

 not supported by objective evidence and received only passing 

 attention. They were soon thrust aside by other interesting if 

 less valuable speculations. 



The development of general knowledge of the animalcules in 

 the early part of the nineteenth century, already referred to 

 in the discussion of the biological relationships and of fermenta- 

 tion, was preparing the way for progress in the problem of disease. 

 In 1834 the contagium 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 



