336 THE PRESERVATION OF HEALTH 



cells belonging to the lowest orders of plants. They pos- 

 sess the power of rapid increase by repeated division of 

 the cells, so that many millions may be produced in a 

 short time from a single one. They are propagated also 

 by means of very minute spores, which may float in the 

 air, mix with the dust of a street or a room, or cling to 

 walls, clothing, or furniture. Only moisture, warmth, 

 and albumin for food are needed for their growth. Prob- 

 ably most (perhaps all) of these tiny organisms have some 

 useful part to play in the infinitely varied operations of 

 nature; but a few varieties are known to be enemies to 

 human life and health. Certain sorts of bacteria are 

 always concerned in the putrefaction of organic matter, 

 that is, as they multiply they break up the complex com- 

 pounds in vegetable and animal substances, and reduce 

 them to simpler chemical forms which may again be used 

 as food for plants. Thus dead animal and vegetable mat- 

 ter is being constantly oxidized by the bacteria found 

 everywhere in the soil, and rendered fit again to support 

 life. In this they of course minister to man's welfare, 

 although the process of decay is attended by the pro- 

 duction of deadly poisons, which may enter the body and 

 cause disease. 



486. Bacteria within the Body. The dry speck of dust 

 which is the spore, or germ, of a disease may, while it 

 remains dry, exist for an uncertain number of years as an 

 inert, harmless bit of matter. It may be subjected for 

 weeks to cold many degrees below zero, and even for a 

 short time to heat above that of boiling water, without 

 destruction of its vitality. It may then fall upon a moist 

 bit of albumin in the air passages of a man, or in the 

 alimentary canal, or in blood or lymph through a broken 

 surface, grow with wonderful rapidity, and swiftly poison 



