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A cluster of Escherichia coli bacteria magnified 10,000 times.

A micro-organism or microbe is an organism which is microscopic, which means so small that people cannot see them with the naked eye. The study of microorganisms is called microbiology.

Micro-organisms include bacteria, fungi, archaea, protists and viruses, and are among the earliest known life forms. The first of these four types of micro-organisms may either be free-living or parasitic. Viruses can only be parasitic, since they always reproduce inside other living things.