Q: Coronavirus is said to kill a larger percentage of those who catch it than the flu, but the flu is said to kill more people overall. Which disease is more deadly? The news media says coronavirus is deadlier. Is that an accepted technical usage?
A: As far as we can tell, the word “deadly” doesn’t have a technical sense that differs from its usual meaning.
We’ve found only one technical reference with an entry for the adjective. The Merriam-Webster Medical Dictionary says it means “likely to cause or capable of causing death.”
That’s pretty much the same definition given in any standard dictionary. Merriam-Webster Unabridged, for example, defines the term in its medical sense as “tending to produce death: productive of death.”
So “deadly” can refer to either the overall number of people killed by a disease or the percentage of infected people who die of it. Since the term can be used both ways, we think writers should clearly indicate which sense is being used when comparing the deadliness of two diseases, such as coronavirus and influenza.
Without a vaccine and adequate public-health measures, coronavirus may turn out to be deadlier than the latest influenza strains in both ways. We assume you’ve seen the recent report by the COVID-19 Response Team at Imperial College in London.
Etymologically, “deadly” comes from adding -lic (an Anglo-Saxon version of the suffix “-ly”) to the Old English noun déad. The usage is ultimately derived from the reconstructed prehistoric root dheu- (to die), says The American Heritage Dictionary of Indo-European Roots.
When “deadly” showed up in Old English, it meant “causing death, or fatal injury; mortal, fatal,” the Oxford English Dictionary says. The earliest OED citation, which uses deadlicne (the accusative, or objective, form of the adjective), is from a ninth-century translation of a Latin history:
“Forbræcon Romane heora aþas … and þær deadlicne sige geforan” (“The Romans broke those pleasant oaths … and carried out their deadly victory”). From an anonymous translation, circa 893, of Historiarum Adversum Paganos (History Against the Pagans), a fifth-century work by Paulus Orosius.
In the late 14th century, the sense of the adjective widened to include something “having the property or capacity of causing death or fatal injury,” according to the OED.
The first citation is from a Middle English sermon by John Wycliffe, written around 1380: “Dedli drynke, ȝif þei taken it … anoieþ hem not” (“Deadly drink, if they have taken it … knoweth them not”).