168 MICROBES, FERMENTS, AND MOULDS. 
malaria only prevails in marshy countries when 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 
different kinds have been assumed to explain the nature 
and progress of great epidemics, such as cholera, yellow 
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 :— 
“Tn the summer and autumn of 1878 the town of 
Spires was visited by cholera, which was limited to 
