Dictionary.net

any organism of microscopic size

Any microscopic form of life; - particularly applied to bacteria and similar organisms, esp. such are supposed to cause infectious diseases.

Any microscopic form of life; - particularly applied to bacteria and similar organisms, esp. such are supposed to cause infectious diseases.

A very tiny organism; a term for microbe.

Minute organism. See Schizomycetes.

Any minute organism, whether animal or vegetable. A general term for microscopic organisms. See also Bacterium, Bacillus, coccus, Micrococcus, Protozoa, etc. [Gr.]