A microorganism, or microbe, is an organism (form of life) that is microscopic (too small to be seen by the unaided human eye). Microorganisms can be bacteria, fungi, archaea, or protists, but not viruses and prions, which are generally classified as non-living. Microorganisms 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 or even a tiny amount of moisture, including hot springs on the ocean floor, deep inside rocks within the earth's crust, on the human skin, in a cow's stomach, and inside a sponge used for washing dishes. Many microorganisms are critical to nutrient recycling in ecosystems as they act as decomposers, while others living in nodules on the roots of someplants convert nitrogen from the air to a form usable by plants. Microorganisms multiply rapidly under good growth conditions often contributing benefit to the larger plant or animal host organism and existing in healthy dynamic balance with other microorganisms and the host organism. At times, however, pathogenic microbes can invade larger organisms, override that organism's defenses, and cause disease.
Microorganisms also are well-known as 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 considered microorganisms. No clear examples of archaean pathogens are known (Eckburg 2003), although a relationship has been proposed between the presence of some methanogens and human periodontal disease (Lepp et al. 2004).
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