Micro-organisms are the organisms which are so small in size that we cannot see them with naked eyes. Eg- bacteria, fungi, algae etc.
The study of micro-organisms is called microbiology. They were discovered in 1674 by Antonie Leeuwenhoek

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They are organisms which are so small that they cannot be seen by the naked eye. They need to be seen by a microscope. For eg- Amoeba, etc.