What is defined as an organism that is visible only with a microscope?

Microscopic organisms are defined as those that cannot be seen with the naked eye and require a microscope to be observed. This category includes various types of bacteria, viruses, protozoa, and some algae and fungi. Microscopic organisms can be important in numerous fields, including biology, medicine, and environmental science, where they play significant roles in health, disease, and ecosystem dynamics.

The term "pathogen" refers specifically to microorganisms that can cause disease, but not all microscopic organisms are pathogens. The word "perennial" relates to plants that live for more than two years, which does not pertain to the visibility of organisms. "Pesticide" is a substance used to kill pests and is not a descriptor for the visibility of organisms. Thus, the definition of an organism that is only visible with a microscope aligns perfectly with the term microscopic.