408 



GENERAL SCIENCE 



FIG. 359. Milk-souring 

 Bacteria. 



which are active in souring milk, making vinegar, pre- 

 paring plant food, and causing the decay of obnoxious 

 dead matter (Figure 359). On the other hand, when 

 bacteria cause the decay of meats, 

 fruits, and foods of all kinds, and the 

 rotting of wooden appliances useful 

 to man, we call them harmful (Fig- 

 ure 360). 



If the single-celled plant or bac- 

 terium is healthy, it grows quite 

 rapidly and divides into two cells, 

 these two dividing in turn, and so 

 on. We can easily see that if this 

 division were kept up the number of bacteria would soon 

 be overwhelming. There is not enough space or food 

 for all, however, and each 

 bacterium has to struggle 

 for existence. Neverthe- 

 less, as it is, the number is 

 countless (Figure 361). 



Germs. Certain bac- 

 teria and a few one-celled 

 animals (protozoa) are 

 able to live and grow as 

 parasites in the bodies of 

 man. These are called 

 disease germs. Every 

 disease is caused by a 

 particular kind of germ. 

 .When bacteria or germs 

 are easily transferred from one person to another, the 

 disease is said to be infectious. 



The fact that germs cause contagious diseases was not 



FIG. 360. A Bit of Decaying Meat. 



magnified, showing the bacteria 

 that cause its decay. 



