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Bacteria are small single-celled organisms. Bacteria are found almost everywhere on Earth and are vital to the planet's ecosystems. Some species can live under extreme conditions of temperature and pressure. The human body is full of bacteria, and in fact is estimated to contain more bacterial cells than human cells. Most bacteria in the body are harmless, and some are even helpful. A relatively small number of species cause disease.

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A kingdom of organisms (equal in rank to the Plant Kingdom or the Animal Kingdom), defined technically as a parasitic or saprobic, filamentous or single-celled eucaryotic organism, devoid of chlorophyll and characterized by heterotropic growth, production of extracellular enzymes. Fungi may cause indoor air quality problems through the dissemination of conidia, spores, toxins, or cell wall constituents.

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A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat. A virus cannot replicate alone; instead, it must infect cells and use components of the host cell to make copies of itself. Often, a virus ends up killing the host cell in the process, causing damage to the host organism. Well-known examples of viruses causing human disease include AIDS, COVID-19, measles and smallpox.

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