A microorganism (also called microbe) is an organism that is so small that it is invisible to the naked eye. These are bacteria, small fungi (for example, yeast) and algae (for example, diatoms), protozoa, and viruses.
Microorganisms are found everywhere in nature, since they have adapted to almost any environment, including the poles, deserts, geysers, and the bottom of the deep sea. Some are known to survive prolonged time in vacuum. (Does this belong here?)
Microorganisms can be helpful in recycling other organisms' remains and waste products, or when employed in biotechnology, e.g., for brewing and bakery. They can also be harmful as pathogens when, as parasites, causing infections.