What organism can only be seen with an electron microscope?

Viruses are an entirely different category of microorganisms that cannot be viewed with a standard light microscope due to their extremely small size, often ranging from 20 to 300 nanometers. This size is significantly smaller than even the smallest bacteria, making traditional optical microscopy inadequate for visualizing them. Electron microscopes, which use beams of electrons rather than light, can achieve much higher resolutions, allowing scientists to observe the intricate structures of viruses.

In contrast, protozoa, bacteria, and fungi are larger organisms that can be observed with light microscopes. Protozoa are single-celled eukaryotic organisms, bacteria are single-celled prokaryotes, and fungi can include both unicellular organisms like yeasts and multicellular organisms like molds. All of these can typically be seen with standard light microscopy due to their larger size compared to viruses. Thus, understanding that viruses require electron microscopy for visualization highlights their unique nature among microorganisms.