Chapter II 
What is a 
microbe? 
THE MICROBE 
M ICROBES, as we have noticed, are so 
small that they cannot be seen with the 
naked eye, but only with a microscope that 
enlarges things a thousand or more times. A 
microbe is the smallest living thing that we 
know. But how small is that? It will help to 
give us an idea if we say that the point made 
by the period mark at the end of this sentence 
would cover about a quarter of a million mi¬ 
crobes of average size. They vary a great deal 
in size, however. There would be about 25,000 
of the averaged sized ones to an inch; of the 
smallest ones, 50,000 to an inch; while of the 
largest ones, only about 1,000 to an inch. 
We know that there are other microbes which 
are even tinier; in fact, so small that even the 
strongest microscope, enlarging three or four 
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