DISEASE GERMS 
77 
It is believed by some scientists that the commonly 
prevalent species of bacteria, harmless under ordinary 
conditions, may change their character when settled 
in thickly crowded centers of population where dark¬ 
ness, dampness, bad air, insufficient or poor food make 
filthy habits of life. If these then gain access to human 
tissues they may develop disease-producing power and 
be carried far and wide. In this- way cholera, the 
“plague,” and similar diseases, beginning in countries 
or sections of cities where human beings herd together 
with no pretense of cleanliness, are carried across seas 
and continents. This would show how necessary to 
the physical health of the 
world is the purification of 
“the slums,” whether these 
occur at home or abroad. 
Other bacteriologists denv 
... . . ' FIG. 42. Typhoid Bacillus Show- 
this hypothesis, but how- ing the Many Cilia. 
... (After Sedgwick and Wilson.) 
ever the disease germs may 
have developed their evil ways, they never, so far as 
we know, reform of their own accord and become 
harmless, although unfavorable conditions may 
weaken their power or virulence. 
The bacteria which are the cause of typhoid, diph¬ 
theria, or tuberculosis make a specialty of this work. 
The true parasitic disease germs affecting man must 
have human beings in which to propagate with any 
degree of success; so the human body is the chief 
natural breeding ground of contagious disease germs . 
Origin 
of Disease 
Germs' 
Breeding 
Ground 
for Germs 
