What organisms are microorganisms?

Microorganisms, (microbes) – the name of a collective group of living organisms that are too small to be visible to the naked eye (their characteristic size is less than 0.1 mm). The composition of microorganisms includes both non-nuclear (prokaryotes) and eukaryotes: bacteria, some fungi, archaea, protists, but not viruses, which are usually isolated in a separate group. Most microorganisms consist of a single cell, but there are multicellular microorganisms. The study of these organisms is the science of microbiology.