microorganism

An organism or infectious agent of microscopic or submicroscopic size, especially a bacterium or protozoan.

A microscopic organism, as a bacillus, bacterium, or vibrio; a microbe; a microzoary.

Any microscopic form of life; a form of life too small to be seen by the naked eye; — particularly applied to bacteria, protozoa, yeasts, and similar organisms, esp. such are supposed to cause infectious diseases.

An organism that is too small to be seen by the unaided eye, especially a single-celled organism, such as a bacterium.

any organism of microscopic size

any organism of microscopic size