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A microorganism (also spelled as microrganism) or microbe is an organism that is microscopic (too small to be seen by the human eye). The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea or protists, but not viruses and prions, which are generally classified as non-living. Micro-organisms are generally single-celled, or unicellular organisms; however, there are exceptions as some unicellular protists are visible to the average human, and some multicellular species are microscopic.