One hundred years ago, the world was in the grip of two deadly calamities. The first was the Great War, and the second was the Spanish flu pandemic of 1918. Both took the lives of hundreds of thousands of otherwise healthy human beings, but the Spanish flu was the deadlier of the two. It was also a misnomer.
The Allied and Central Forces were both losing soldiers to this disease, which the best minds in science didn’t know how to fight. (They didn’t even know it was a virus.) Naturally, neither side wanted the other to know how that outbreaks were decimating their ranks, and so they kept this information under wraps.
Neutral Spain, though, had no qualms about releasing information about a new medical threat that was ravaging its population. Because so many reports of this strain of flu came from Spain, it came to be called Spanish influenza.
Truth is, the first known outbreaks occurred on military bases in Kansas. The first wave, in the spring of 1918, was the least worrisome of what would eventually be three waves of flu by the end of the next spring. After the soldiers there recovered, they were shipped off to Europe to fight the war and — it turned out — spread the Spanish flu throughout Europe, creating a pandemic.
The word pandemic comes from the Greek pan- “all, whole” + demos “people” — the same root that gives us democracy and demagogue. A pandemic is an infectious disease that spreads very rapidly and without regard to geographic, geopolitical, or genetic borders. That is, it can spread to all people, hence the pan-.
The related and older word epidemic describes something slightly different. The word contains the same demos, but its prefix epi- means “among, upon.” Like a pandemic, an epidemic is an infectious disease that spreads very rapidly, but it is limited to a specific population.
Had those soldiers been kept home in Kansas, the Spanish flu pandemic might have been “only” an epidemic in the United States — and the last year of World War I might have played out differently. (Attention writers of alternative histories!)
I’ve had a difficult time in the past keeping pandemic and epidemic clear in my mind, but here’s the mnemonic device I came up with: I equate pandemic with Pangaea, the theoretical supercontinent that comprised more or less all the land on the planet (the word literally means “all earth”) millions of years ago. Pangaea is “all the land,” and pandemic is “all the people.” That leaves epidemic to describe a fast-spreading disease with a more limited scope. (Indubitably, editors with more experience in medical editing have better mnemonic devices.)
You should also note that epidemic is used metaphorically more often than pandemic is to describe some usually undesirable thing that seems to be spreading rapidly: an epidemic of skinny jeans, for example. Or an epidemic of language podcasts. It certainly pairs well with the metaphorical use of viral memes.
A third related word is endemic, en- meaning “in.” Something that is endemic is characteristic of or prevalent in a particular population. It doesn’t necessarily have to indicate something of an infectious or even medical nature, but it can. An endemic disease is one that is always present in the population or geographic area, though there might not necessarily be an outbreak. For example, Japanese encephalitis is a vaccine-preventable disease that is endemic to Southeast Asia, where it is carried by mosquitoes; the disease is always there, and unvaccinated people are always at risk of getting it, but you can’t catch the disease anywhere else.
Endemic diseases also aren’t fast-spreading the way epidemics and pandemics are. If an outbreak of an endemic disease were to start spreading rapidly, it would be called an epidemic.
As for the Spanish flu, information about the Great War has far overshadowed reports about the flu outbreak, both in news reports then and in history textbooks now. During much of the second half of 1918, schools and stores were shut down; when hospitals filled, the infected died in their homes or in the streets; morticians ran out of coffins. Because of a lack of a central reporting agency at home and the overlap with and censorship of war casualty reports abroad, we will never know for certain how many deaths were caused by the Spanish flu. Estimates are that as any as 500 million people were infected, and that 10 to 20 percent of those infected died. It very likely killed more people than the bubonic plague.