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WORLD OF INVISIBLE LIFE 
How did the 
discovery of 
microbes help us? 
swept the cities, sometimes killing nearly every¬ 
body. In the years 1348 and 1349, one-quarter 
of the population of Europe, or about 25,000,000 
people, died of this disease. 
It was only after the discovery of microbes 
and the studying of their actions that people 
began to learn the right way of fighting sick¬ 
ness. The part that microbes play in causing 
disease was found out only about fifty years 
ago. Though the existence of microbes had 
been known for a hundred years before, it was 
a great Frenchman named Louis Pasteur (liv¬ 
ing from 1822 to 1895), who first made clear 
to the world how microbes cause things to de¬ 
cay and ferment. Then, in 1876, Robert Koch, 
a German, showed that one kind of microbe was 
the cause of a certain disease, called splenic 
fever, in cattle. Koch also found a way to sepa¬ 
rate a single kind of microbe from a group con¬ 
taining many kinds, and to grow it in a pure 
state in the laboratory. 
The work of these two men was the begin¬ 
ning of the scientific study of microbes, show- 
