Microorganisms vary significantly in size, typically requiring microscopy for visualization. For comparison, a standard plant or animal cell is ${}10$ to ${}100$ micrometers (µm), while a typical bacterial cell is about ${}1$ µm. Viruses are even smaller, with polio and flu viruses measuring around ${}50$ nanometers (nm) and ${}100$ nm respectively. Light microscopes can resolve objects larger than ${}100$ nm, but visualizing smaller structures, like most viruses or molecules down to ${}1.5$ nm, requires an electron microscope.