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Microorganisms were the first living creatures on earth. It is estimated there are a total of 5 nonillion microorganisms on earth (5x1020). You will find nearly 10 million a millilitre of ocean water and 40 million in a gram of soil. They are also ubiquitous on the human body.
Microorganisms include bacteria, fungi, archaea or protists and viruses. The first four types of microorganisms may be either free-living or parasitic. Viruses, however, always reproduce inside other living things.