What is a microorganism?

A microorganism is defined as a small living thing that can only be seen with a microscope due to its tiny size. This broad category includes various types of organisms, such as bacteria, viruses, fungi, and protozoa, which range in complexity and function. The significance of microorganisms lies not only in their role in causing diseases (as indicated in other choices) but also in their presence in various environments, where they can have beneficial effects, such as in digestion and nutrient cycling.

Understanding microorganisms is crucial in the field of healthcare as it informs practices related to infection control, hygiene, and the use of antibiotics. The other options contain specific examples or subsets of microorganisms but do not capture the full definition or scope of what constitutes a microorganism.