Final answer:
Viruses are much smaller than bacteria, typically ranging from 20-300 nm in size, whereas bacteria are usually 0.5-5.0 µm. Viruses require electron microscopy to be observed and are not viable on their own, needing a host cell to reproduce.
Step-by-step explanation:
Compared to bacteria, the size of viruses is generally much smaller. A virus, which is a sub-microscopic infectious agent, can range in size from about 20-300 nanometers (nm). In contrast, prokaryotes, such as bacteria, are typically 0.5-5.0 micrometers (µm) in length. For instance, a small virus like the one that causes foot and mouth disease is just 21 nm in diameter, while bacteria generally measure 1,000 to 2,000 nm (micromicrons). Even giant viruses, which are larger than typical viruses, still tend to be smaller or comparable to bacterial cells.
Since viruses are not visible under the light microscope, we rely on electron microscopy to observe these tiny agents. Viruses must infect a living cell to reproduce, and their small size is an adaptation that allows them to do so with relative ease. Significant differences in size are one of the main distinctions between viruses and bacteria, which are complete cells capable of independent reproduction.