366 SCIENCE OF HOME AND COMMUNITY 



systems, and yet while one person becomes sick, others may 

 be unaffected, or very slightly so. One person may have 

 disease germs in his system and not be affected by them, 

 while another person may receive these germs from the first 

 person and come down with the disease. 



Characteristics of bacteria. In order to understand how 

 these parasites cause disease and what may be done to 

 control them, we need to know something about their general 

 activities. 



Size. Bacteria are the smallest living organisms that have 

 been discovered with the microscope. They vary in size from 

 a ten-thousandth to a hundred-thousandth of an inch in 

 diameter. There are probably others so much smaller that 

 they cannot be seen even with the aid of the microscope. 

 It would take about 1 500 bacteria of average size placed end 

 to end to reach across the head of a common pin. It has 

 been estimated that a pint can would hold over two hundred 

 billion bacteria. But although they arc so extremely small, 

 these little plants play a very important part in man's life 

 on account of their frequent occurrence and power of rapid 

 reproduction. While we cannot see bacteria without the aid 

 of the microscope, the effect of their action in masses is 

 evident all around us, as in the souring of milk, the spoiling 

 of food, and the decay of refuse and other organic matter. 



Multiplication. The great abundance of bacteria depends 

 upon the remarkable rapidity with which they grow and 

 form new bacteria. Their means of multiplication is a 

 process called division. A bacterium divides into two 

 similar parts, each part growing meanwhile till it is as large 

 as the first bacterium. In a short time, sometimes in twenty 

 minutes, these two divide to make four ; in another twenty 

 minutes these four divide to make eight, and so on, each 

 division doubling the number of bacteria. At this rate, 

 the descendants of a single bacterium would amount in ten 

 hours to about a billion. Bacteria do not continue to grow 



