Any organism so small that it can be seen only with a microscope is a microorganism. Microorganisms are found in three of the kingdoms: protoctists, fungi and prokaryotes. All prokaryotes are microorganisms. When light microscopes were invented three hundred years ago, people saw microorganisms for the first time and saw them only as small specks. Even when light microscopes improved, it was still difficult to see much inside a microorganism. When electron microscopes were invented in the 1920s, we learnt a great deal more about microorganisms.
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