Microorganism

A microorganism (from the Greek: μικρός, mikros, "small" and ὀργανισμός, organismós, "organism") is a microscopic living organism, which may be single celled or multicellular. The study of microorganisms is called microbiology, a subject that began with the discovery of microorganisms in 1674 by Antonie van Leeuwenhoek, using a microscope of his own design.

Microorganisms are very diverse and include all bacterial, archaean and most of the protozoan species on the planet. This group also contains some species of fungi, algae, and certain animals, such as rotifers. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses (and viroids) as microorganisms, but others consider these as nonliving.