ARTICLE

Microorganism

A microorganism (also spelled as microrganism) or microbe is an organism that is microscopic.

The study of microorganisms is called microbiology.
Microorganisms include bacteria, fungi, archaea or protists, but not viruses and prions, which are generally classified as non-living.

Most microorganisms are single-celled, or unicellular, but some are microscopic, and some unicellular protists are visible to the average human. Microorganisms live almost everywhere on Earth where there is liquid water, including hot springs, on the ocean floor, and deep inside rocks within 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 cause diseases that kill millions of people every year. Microorganisms can be found almost anywhere in the taxonomic organization of life on the planet.

Bacteria and archaea are almost always microscopic, while a number of eukaryotes are also microscopic, including most protists and a number of fungi.
Viruses are generally regarded as not living and therefore are not microbes, although the field of microbiology also encompasses the study of viruses. Habitats and ecology Microorganisms are found in almost every habitat present in nature.

Even in hostile environments such as the poles, deserts, geysers, rocks, and the deep sea, some types of microorganisms have adapted to the extreme conditions and sustained colonies; these organisms are known as extremophiles.

Extremophiles have been isolated from rocks as much as 7 kilometres below the earth's surface, and it has been suggested that the amount of living organisms below the earth's surface may be comparable with the amount of life on or above the surface.
Extremophiles have been known to survive for a prolonged time in a vacuum, and can be highly resistant to radiation, which may even allow them to survive in space.

Many types of microorganisms have intimate symbiotic relationships with other larger organisms; some of which are mutually beneficial (mutualism), while others can be damaging to the host organism (parasitism).
If microorganisms can cause disease in a host they are known as pathogens. Microorganisms are vital to humans and the environment, as they participate in the Earth's element cycles such as the carbon cycle and nitrogen cycle, as well as fulfilling other vital roles in virtually all ecosystems, such as recycling other organisms' dead remains and waste products through decomposition.

Microbes also have an important place in most higher-order multicellular organisms as symbionts. Microorganisms are the cause of many infectious diseases.
The organisms involved include bacteria, causing diseases such as plague, tuberculosis and anthrax; protozoa, causing diseases such as malaria, sleeping sickness and toxoplasmosis; and also fungi causing diseases such as ringworm, candidiasis or histoplasmosis.

However, other diseases such as influenza, yellow fever or AIDS are caused by viruses, which are not living organisms and are not therefore microorganisms.
As of 2007, no clear examples of archaean pathogens are known, although a relationship has been proposed between the presence of some methanogens and human periodontal disease.