DISEASE GERMS 77 



It is believed by some scientists that the commonly Origin 

 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 deny 



FIG. 42. Typhoid Bacillus Show- 

 thlS hvDOtheSlS, 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- Breeding 

 theria, or tuberculosis make a specialty of this work. forGermi 

 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. 



