William Burroughs called language a virus as early as 1962, in The Ticket That Exploded, but it’s not clear whether he ever actually wrote the words “Language is a virus from outer space.” Laurie Anderson thought he did when she sang the same words in 1986, in any case, and since then the sentence itself has, arguably, gone viral.

The word virus came into Middle English from Latin, where it meant slime, venom, or poison. In the eighteenth and nineteenth centuries, doctors used it to mean any infectious agent, applying it to smallpox, which is a virus according to modern understanding, but also to syphilis and typhus, which aren’t. (Edward Jenner, who figured out that people who got cowpox wouldn’t get smallpox, discovered vaccination—literally “cow-action”—without knowing why it worked or what a virus was.)

Decades later we recognize viruses as small arrangements of big molecules, with DNA or RNA at the core. Unlike other infectious agents, viruses cannot reproduce on their own; they hijack a host cell and cause it to copy their DNA or RNA, in turn instructing the host cell to make the rest of their few parts. Some viruses, like Ebola, do so very rapidly, until the host cells burst; others hang out in their hosts and reproduce slowly, even harmlessly. If you wanted to invent a virus that would stick around forever, your best bet would be something less like Ebola, which kills hosts and can prompt quarantines, and more like the common cold, whose hosts generally survive to keep passing it on.

Because viruses are, in a sense, information— genetic instructions adapted to some other, larger thing that must work to transmit them—they make an especially good analogy for the spread of ideas, or words, or media properties. Rebecca Black’s song “Friday” didn’t spread itself; lots of people had to view it and share it. Indeed, like a work of art, a virus is only ambiguously or figuratively a living thing. “Are you too deeply occupied,” Emily Dickinson asked a correspondent, “to say if my Verse is alive?” Dickinson’s verse continues to live, as a virus does, because we reproduce it. Whatever the verse or the video or the meme does to us causes at least some of us to pass it on.

Words can work the same way (see meme, for instance, or...