A microorganism or microbe is an organism that is so small that it is invisible to the naked eye. Microorganisms are single-celled or unicellular organisms. However, some unicellular protozoa are visible to the naked eye and some multicellular species are microscopic. The study of microorganisms is called microbiology.
Microorganisms are found everywhere in nature even in hostile environments like poles, deserts, geysers, rocks and the deep-sea.