Microorganisms.

Microorganisms are very small organisms that generally cannot be seen without a microscope. These tiny organisms live everywhere, including within the human body. Microorganisms may be prokaryotic or eukaryotic. Prokaryotic microorganisms (or prokaryotes) include bacteria and archaea, which are single-celled organisms that lack a membrane-bound nucleus. Eukaryotic microorganisms (or eukaryotes) may be unicellular or multicellular and include some plants, animals, and fungi. Eukaryotic microorganisms have cells that contain a nucleus.