A microorganism (also spelled as microrganism) or microbe is an organism that is microscopic (too small to be visible to the human eye). The study of microorganisms is called microbiology. Microorganisms can be bacteria, fungi, archaea or protists, but not viruses and prions because they are generally classified as non-living. Micro-organisms are often described as single-celled, or unicellular organisms; however, some unicellular protists are visible to the human eye, and some multicellular species are microscopic.
Microorganisms live almost everywhere on earth where there is liquid water, including hot springs on the ocean floor and deep inside rocks within the earth's crust. Microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers. As some microorganisms can also fix nitrogen, they are an important part of the nitrogen cycle. However, pathogenic microbes can invade other organisms and therefore cause disease.
