168 MICROBES, FERMENTS, AND MOULDS. 



malaria only prevails in marshy countries wlien the 

 marshes are partially dry, and especially in summer. 

 In order to make such country healthy, the marshes 

 must be completely dried and filled up, and then 

 transformed into cultivated ground. So, again, the 

 river valleys in France only become unhealthy when 

 the stream returns to its bed, leaving the adjoining 

 meadows transformed into marshes, which gradually 

 dry up and send forth into the air a host of spores, 

 produced by the schizophyta deposited by the water. 

 Finally, great excavations of earth diffuse through the 

 atmosphere the dormant spores brought thither by rain, 

 and remaining in a desiccated state in the soil. 



In many cases, the intervention of two microbes of 

 difierent kinds have been assumed to explain the nature 

 and progress of great epidemics, such as cholera, yeUow 

 fever, and typhoid fever. This is termed by Nageli the 

 diblastic theory (or that of two producing agents of 

 disease). Thus the microbe of malaria, or intermittent 

 fever, which is not contagious, often predisposes the 

 patient to receive the attacks of another zymotic 

 disease, such as cholera or typhoid fever. The two 

 microbes may subsist simultaneously in the human 

 frame, and their joint action may weaken the organism 

 at the expense of which they live and multiply. 

 Numerous cases might be cited to support this theory, 

 and the following examples may be given : — 



"In the summer and autumn of 1873 the town of 

 Spires was visited by cholera, which was limited to 



