“The earth is dominated by microorganisms. It can be difficult to appreciate this fact for obvious reasons that these forms of life are invisible to the unaided eye. We see plants and animals and interact with them in a deliberate fashion, and for most of human history we had no proof that anything smaller than insects existed. The Roman philosopher Lucretius edged close to the truth with his conjecture that ‘certain minute creatures . . . enter the body through the mouth and nose and cause serious disease’, His musings began to make sense after the invention of the microscope in the 1600s. The numbers of microbes are staggering. Tens of millions of viruses live in a pinch of soil; a drop of seawater contains 500,000 bacteria and tens of millions of viruses, and a hundred trillion bacteria swarm inside the human gut. Every macroscopic organism and every inanimate surface is coated with microbes; microbes grow around volcanoes and hydrothermal vents; they live in blocks of frozen blocks of sea ice, in the deepest oceans, and thrive in the sediment on the seafloor.”

“Our bodies are home to an incredible range of microorganisms: molecular analysis has revealed 2,368 species of bacteria that live in the human navel.”

“. . . that there may be tens or even hundreds of millions of species of bacteria, and other microorganisms is an important theme in the struggle to develop objective measurements of biodiversity.”

From Microbiology: A Very Short Introduction by Nicholas P. Money