Viruses as a group are the smallest microbes. Specific kinds of
viruses range in size from a few dozen nanometers up to about
1,000 nanometers or 1 micrometer. Bacteria and archaea are
generally the second smallest microbes as a group. But some
types of bacteria—Chlamydia, for example—are about as small as
the largest viruses. And there are a couple of monster
bacteria—Thiomargarita namibiensis and Epulopiscium
fishelsoni—that are actually large enough to see without a
microscope. These two bacteria are even larger than many single-
celled protozoa, which are generally considered among the
largest of microbes.