Which organisms are considered the smallest?

Viruses are considered the smallest organisms because they are significantly smaller than bacteria, fungi, and protozoa. In fact, viruses typically range from about 20 to 300 nanometers in size, whereas bacteria can range from 0.5 to 5 micrometers, making them much larger. Fungi and protozoa are also larger, as most fungal cells and protozoa can be seen under a light microscope and range from 2 to 10 micrometers or more in size.

Viruses are unique because they are not classified in the same way as other cellular organisms; they consist of genetic material encapsulated in a protein coat and require a host cell to replicate. This characteristic contributes to their classification as acellular and indicates their size disparity when compared to the other organisms listed. Understanding this size variation is essential in fields like sterile processing, where the control of various microorganisms is critical for maintaining a sterile environment.