42 INFECTIOUS AND PARASITIC DISEASES. 



to appear and disappear. What the circumstances 

 are that lead to this change will be explained in the 

 third and fourth chapters. 



In size bacteria are the smallest living 

 Size. things that we can see with the modern 



microscope. In masses they are readily 

 seen with the unaided eye as moist, slimy, or dry films 

 floating upon the surface of foul fluids or water, or 

 covering decomposing animal or vegetable matter. 

 Some of them produce beautiful colors; others have the 

 peculiar property of producing phosphorescence; still 

 others cause the stench of decomposition. 



Bacteria vary in size between five-tenths of a micro- 

 millimeter and twenty to forty micro-millimeters, and 

 as a micro-millimeter is the one-thousandth part of a 

 millimeter (about one-twenty-five-thousandth of an 

 inch), only a vague conception of their minuteness can 

 be formed unless one has had experience in working 

 with microscopic forms of life. 



What bacteria lack in size they make up 



in the astonishinoj capacity that they 



CATION. '^ . . , . . -r- 1 



possess of rapid multiplication. Each 

 bacterium consists of a single cell only (unicellular), 

 which under favorable circumstances produces count- 

 less other cells. This is accomplished by simple 

 division (fission) of the whole cell body. The two 

 individuals formed out of the first one, begin dividing 

 into four before complete separation has taken place; 



