Micro-organism

Any living thing too small to be seen without the aid of a microscope, particularly a single- celled creature. There are both animal and plant forms of micro-organism. They include protozoa, bacteria, various kinds of algae, and such fungi as yeasts and moulds. Viruses are sometimes regarded as micro-organisms, but many biologists consider them to be non living things. A great number of micro-organisms are disease-inducing parasites, but many more are useful to other life forms.