Microbe is the shortened form of microorganism that describes any organism that is too small to be seen without magnification. Unlike the word germ, which names “the causative agent or source of a disease, esp. an infectious disease” (OED Online 2020, “germ”), microbe refers to a broader range of microorganisms like bacteria, fungi, and viruses, whether or not they cause disease. In the scientific, medical, and popular lexicon, microbe can describe anything from the “helpful” microbes living in the human digestive tract to the viruses that cause influenza or the common cold. Although early microscopists like Robert Hooke and Antonie van Leeuwenhoek observed microscopic forms of life in the seventeenth century, the term microbe did not appear in English until the 1880s (OED Online 2020, “microbe”), when bacteriology emerged as a new scientific discipline after the germ theory of disease, which posited that a specific microorganism causes a specific disease, gained traction in scientific and medical discourse. Despite the existence of earlier theories of contagion (see “Contagion”), it was not until scientists like Louis Pasteur and Robert Koch established laboratory procedures to isolate and identify microorganisms that they could generate a consistent theory through a series of experimental proofs. Bacteriological...