CAUSATION Of DISEASS 25 



only of the pus but also of the nasal discharges, saliva, 

 urine, etc., of gland ered horses. Prevost's discovery of the 

 cause of grain rust (Pucclnia (/rammis) in 1807 was the first 

 instance of an infectious disease of plants shown to be due 

 to a microscopic plant organism, though not a bacterium in 

 this case. 



In 1822 Gaspard showed the poisonous nature of material 

 from infected wounds by injecting it into animals and caus- 

 ing their death. Bearing on the "contagium vivum" theory 

 was the rediscovery of the " itch mite" {Sarcoptes scabiei) by 

 Renucci (1834), an Italian medical student. This had been 

 declared several hundred years before but had been lost 

 sight of. Chevreuil and Pasteur, in 1836, showed that putre- 

 faction did not occur in meat protected from contamination, 

 and suggested that wound infection probably resulted from 

 entrance of germs from without. Bassi, investigating a dis- 

 ease of silkworms in Italy, demonstrated that a certain 

 mold-like fungus {Botrytis bassiana) was the cause in 1837. 

 This was the first instance of a microscopic vegetable organ- 

 ism proved to be capable of causing disease in an animal. 



Boehm, in 1838, observed minute organisms in the stools 

 of cholera patients and conjectured that they might have a 

 causal connection with the disease. The fungous nature of 

 Favus, a scalp disease, was recognized by Schonlein in 1839, 

 and the organism was afterward called "Achorion schoen- 

 leinii." Berg, in 1839-41 , showed that Thrush is likewise due 

 to a fungus, "O'idium albicans." 



These discoveries led Henle, in 1840, to publish a work in 

 which he maintained that all contagious diseases must be 

 due to living organisms, and to propound certain postulates 

 (afterward restated by Koch and now known as "Koch's 

 postulates") which must be demonstrated before one can be 

 sure that a given organism is the specific cause of a given 

 disease. The methods then in vogue and the instruments of 

 that period did not enable Henle to prove his claims, but he 

 must be given the credit for establishing the "contagium 

 vivum" theory on a good basis and pointing the way for 

 men better equipped to prove its soundness in after years. 



In 1842-43 Gruby showed that Herpes tonsurans, a form 



