The vast majority of microbes are unicellular and require artificial magnification to be observed. However, there are important exceptions: some unicellular microbes are visible to the naked eye, and some multicellular organisms are microscopic. While an object must measure approximately ${}100$ micrometers (µm) to be visible without a microscope, most microorganisms are significantly smaller. For perspective, a typical microscopic animal cell is roughly ${}10$ µm across, bacterial cells average about ${}1$ µm, and viruses can be up to ${}10$ times smaller than bacteria.