What is the term for any microscopic or submicroscopic size organism?

The term "microorganism" encompasses a wide range of microscopic life forms, including bacteria, viruses, fungi, and certain types of simple algae. This inclusive definition captures organisms that cannot be seen with the naked eye, typically measuring less than 0.1 mm in size. Microorganisms play vital roles in various ecosystems, human health, and can be found almost everywhere, from soil to water to human bodies.

While viruses and bacteria are types of microorganisms, they do not represent the complete scope of organisms that fall under this term. Additionally, "pathogen" specifically refers to microorganisms that can cause disease; however, not all microorganisms are pathogenic. Therefore, "microorganism" is the most accurate and comprehensive term to describe any organism that exists at the microscopic or submicroscopic level.