Viruses must enter and inhabit a living cell in order to reproduce, or to do much of anything. On their own, they are little more than minuscule bits of inert genetic material, so small that they cannot be seen by an ordinary microscope. Once inside another cell, viruses use that cell’s body to make more of themselves. The metaphor of a factory is often used to describe how viruses work—they enter a cell and force its equipment to produce thousands more viruses. But viruses strike me as more supernatural than industrial—they are zombies, or body snatchers, or vampires.
