A very small, single-celled living organism that cannot usually be seen by the naked eye. The most important micro-organisms in medicine are those that cause disease. This ‘pathogenic’ group, however, forms only a small proportion of the enormous number of known microorganisms. The main pathogenic ones are BACTERIA. Others are fungi and RICKETTSIA. Though not true cells, viruses (see VIRUS) are usually classi?ed as micro-organisms. (See also MICROBIOLOGY.)
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