THE MICEOBES OF HUMAN DISEASES. 191 



power (more than 1000 diameters). But they are not 

 invariably present, and it is consequently uncertain 

 whether they are the cause of the disease. From its 

 symptoms and lesions, there is reason to think that 

 the parasite or parasites — for there may be several, 

 according to Nageli's theory — have their seat in the 

 digestive canal. ^ New and sustained researches, carried 

 on in countries where yellow fever prevails, and more 

 methodically conducted, are necessaiy to elucidate this 

 question. 



VII. TyPHOiD AND Typhus Fevees. 



These two diseases may be taken together, since 

 in both the digestive canal is the part chiefly affected. 



Here crowding, the aggregation of men and the 

 human miasmata resulting from it, play the chief 

 part, admitting, as we have already said, that miasma 

 means microbe. We need not, therefore, deny the in- 

 fluence of predisposing conditions, or what is called 

 receptivity for the disease. These unfavourable con- 

 ditions are: physical exhaustion, bad food, youth, 

 mental emotion — all which conditions are allied with 

 human miasmata, the result of crowding in barracks, 

 where typhoid fever prevails; in camps, which are 

 more subject to typhus; and in the badly built 

 houses of our large cities. 



In few diseases is the influence of anti-hygienic 



