Living creatures of microscopical size and relatively simple structure. A term embracing viruses, bacteria, fungi and protozoa.
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 micro-organisms. The main pathogenic ones are bacteria. Others are fungi and rickettsia. Though not true cells, viruses are usually classified as microorganisms.