Without the modern scourge of light pollution, the ancient Greeks and Romans saw more of the heavens than we normally do, and they made stargazing a more central aspect of their culture. Following the number-loving philosopher Pythagoras, Cicero wrote that the intervals of a musical scale echo the intervals between the orbits of our solar system. The Stoics, believers in an interdependent cosmos, looked to the night sky to augur our predestined lives. Of particular interest to all these ancients was the stella erratica, or “errant star,” so called for its shifting location. (Our familiar constellations, by contrast, remain fixed in the firmament.) Romans borrowed a word from the Greeks to denote these celestial strays: planeta, or “planet.”

Derived from the verb “to wander,” the original Greek noun πλάνης was applied to more than just Mars and Saturn—in Euripides’s Bacchae, to take just one example, it refers to a “vagabond” who comes to town. Among the physicians of the ancient world, including Hippocrates himself, πλάνης could also mean “fever,” a pestilence that migrates from person to person. The Romans, of course, had their own words for disease—morbus, pestis—but they adopted this astronomical language in their own medical writings too, using the Latin cognate. In one account, planeta refers to a fever with an “unrestrained onset.” In another, planetae are those illnesses that obey neither finite duration nor predictable prognosis.

According to the lexicographical history of planeta that Fordham’s Matthew McGowan catalogs in the Thesaurus Linguae Latinae, the same ancient word means both a celestial body and a mundane affliction. But with some creative reflection, this paradoxical pairing starts to make a little sense. Both the errant star and the roving illness shatter our attempts at a tidy ordering of the natural world around us and within us. Planets and plagues appear to move according to their own plans, not ours.

This Greco-Roman metaphor hints at the unpredictability we grapple with daily while the coronavirus pandemic continues to elude containment. In the absence of adequate laboratory testing, our methods of diagnosing COVID-19 sometimes remain at the level of errant guesswork—speaking for myself, I now head to the kitchen periodically to sniff aromatic tomatoes and hot sauce to check my sense of smell, hardly the pinnacle of scientific precision. And as billions of people are now learning through strict restrictions on movement, it is mankind that needs to fix itself into immobile constellations when the natural world decides to break out of its familiar orbit.

While we might find the ancient coupling of planets and pestilence jarring, our language has its own startling metaphorical bedfellows. Our word “virus” is a rich example. In ancient Latin texts, a virus is some kind of biological fluid—in Pliny the Elder, it means semen on one page and venom on another. Taking up these Roman antecedents, the English “virus,” as the Oxford English Dictionary reports, originally points to such substances, including venom but also pus and other humors. Only in the first years of the twentieth century does the modern notion of a “virus” emerge—that of a tiny pathogen that invades the living cells of other organisms.

Of course, the linguistic transfer from semen to pus to pathogen aligns with the technological developments that enabled scientists to observe something like a coronavirus in the first place. A 1915 issue of the Lancet hypothesizes that it “is quite possible that an ultra-microscopic virus” belongs among other known disease-causing agents like bacteria, which are significantly larger. As photographs in today’s newspapers confirm with the help of advanced microscopes, that earlier hypothesis has borne out, replacing the image of snake venom with one of geometric capsids.

Even more disorienting is the history of the adjective “viral,” which mere weeks ago brought to mind primarily the phenomenon of internet celebrity. If we find surprising the ancients’ use of the same word to describe planets above and diseases within, I imagine future historians will marvel at the state of language in 2020, when “virality” could simultaneously denote ironic meme culture and a global medical panic.

But these manifold lexicographical histories—both ancient and modern—illustrate for us how language, like biology, is always on the move. We should take our era of pandemic as an arresting reminder that, like a Euripidean vagabond or a cityscape moonrise, so much of our lives wanders in unbidden.