THE MICROBES 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 Nigeli’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 necessary to elucidate this 
question. 
VII. TypHorp anp TypHus FEVERS. 
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 
