Definition of Microbe

Microbe: A very tiny form of life -- microbes include bacteria, fungi, and protozoan parasites -- best visualized under a microscope.

The Microbe is so very small
You cannot make him out at all,
But many sanguine people hope
To see him through a microscope.

(From "The Microbe" by Hilaire Belloc, 1900)

The word "microbe" did not enter English usage until 1881. It was put together from Greek roots "mikros," small + "bios," life = small (form of) life.