The flu has been infecting humans for at least several centuries, with the first clearly documented pandemic striking in 1580. Descriptions of flu-like illness go back even further, possibly to ancient Greece around 412 BC, though confirming influenza from ancient texts is unreliable. What we know for certain is that influenza A viruses have circulated in wild aquatic birds for far longer than humans have existed, and the virus has been jumping into human populations repeatedly throughout recorded history.
The Earliest Written Clues
The oldest candidate for a flu epidemic comes from Hippocrates, who described an outbreak near the Greek colony of Perinthus around 412 BC. He recorded coughs that started near the winter solstice, accompanied by sore throats, difficulty breathing, aches, and several other symptoms. For a long time, historians treated this “Cough of Perinthus” as a single disease and speculated it might have been influenza. But when scholars revisited the text more carefully, they realized Hippocrates was likely describing a cluster of different illnesses circulating at the same time, not one unified epidemic. Retrospectively diagnosing diseases from 2,500-year-old case notes is, as researchers have put it, difficult if not impossible.
The first outbreak that historians confidently identify as influenza swept across Europe, Africa, and Asia in 1580. From that point forward, records become detailed enough to track recurring waves of a respiratory illness that matches what we now call the flu. Between the 1500s and the early 1900s, at least a dozen major flu epidemics hit Europe alone.
Where the Word “Influenza” Comes From
The word itself is Italian. It literally means “influence,” from the medieval Latin influentia, reflecting an old belief that epidemics were caused by the influence of the stars or unfavorable atmospheric conditions. By at least 1504, Italians were using “influenza” to mean an epidemic disease. The name stuck, even after science moved well past astrology as an explanation.
The Virus Lives in Birds
Influenza A viruses, the type responsible for every flu pandemic in modern history, naturally live in wild aquatic birds like ducks, geese, and shorebirds. These birds carry a huge variety of flu subtypes in their intestines, often without getting sick. The World Health Organization considers this reservoir permanent, meaning flu can never truly be eradicated. The virus circulates quietly in bird populations and periodically spills over into pigs, horses, dogs, and humans.
This is why the flu keeps coming back in new forms. When an avian flu virus picks up the ability to infect humans, or when genes from bird and human flu viruses mix together (often inside a pig, which can catch both), the result can be a strain that nobody’s immune system recognizes. That’s the recipe for a pandemic.
How the Virus Keeps Changing
The flu stays one step ahead of your immune system through two distinct tricks. The first, called antigenic drift, is a slow, constant process. Every time the virus copies itself inside a host, small mutations accumulate in its surface proteins. Over time, these tiny changes add up until your antibodies from a previous infection or vaccination no longer recognize the virus well. This is why you can catch the flu more than once in your life, and why flu vaccine formulas are reviewed and updated every year.
The second trick is far more dramatic. Called antigenic shift, it happens when a flu virus undergoes a sudden, major change, acquiring entirely new surface proteins, usually from an animal flu strain. Because most people have zero immunity to the new combination, shift events are what trigger pandemics. They’re rare but devastating when they occur.
A Timeline of Major Pandemics
The 1918 flu pandemic remains the most catastrophic infectious disease event in modern history. Caused by an H1N1 virus with genes of avian origin, it killed at least 50 million people worldwide and more than 550,000 in the United States between 1918 and 1920. Molecular clock analysis suggests the components of this virus were circulating in mammalian hosts, likely pigs and humans, for 2 to 15 years before the pandemic exploded. Some gene segments may have been present in mammals as early as the 1880s.
In 1957, a new H2N2 strain emerged, combining human flu genes with three new gene segments from avian sources. This “Asian flu” pandemic killed about 1.1 million people globally and 116,000 in the U.S. Genetic analysis suggests those avian genes entered human populations 2 to 6 years before the pandemic became visible.
The 1968 pandemic followed a similar pattern. A new H3N2 virus appeared, killing roughly 1 million people worldwide and 100,000 in the U.S., with most deaths among people 65 and older. The H3N2 viruses circulating today are direct descendants of that 1968 strain, still drifting and evolving more than 50 years later.
When Science Finally Caught Up
For most of human history, nobody knew what caused the flu. The virus itself wasn’t isolated until 1933, when British researchers successfully transmitted human influenza to ferrets, proving it was caused by a virus rather than bacteria. Within a decade, that discovery led to the first vaccine. With support from the U.S. Army, Thomas Francis and Jonas Salk developed an inactivated flu vaccine at the University of Michigan, tested it on military personnel, and saw it licensed for public use in 1945.
That 12-year sprint from viral discovery to licensed vaccine was remarkably fast for the era, driven largely by military interest in preventing the kind of devastation the 1918 pandemic had inflicted on troops.
The Flu Today
Seasonal influenza now infects roughly 1 billion people every year worldwide. Of those, 3 to 5 million develop severe illness, and between 290,000 and 650,000 die from respiratory complications annually. Those numbers fluctuate depending on which strains dominate each season and how well the year’s vaccine matches them.
Wild aquatic birds continue to serve as the permanent reservoir for influenza A, which means new pandemic threats will keep emerging. The virus has been reshuffling its genes and jumping between species for millennia. What’s changed is our ability to track it. Global surveillance networks now monitor flu strains year-round in both hemispheres, and vaccine production can be scaled up faster than at any point in history. The flu isn’t going anywhere, but neither is our capacity to respond to it.