Flu has sickened people for centuries, but its cause was only pinned down in 1933, when three British scientists caught the virus in ferrets.1CDC Museum. Discovering the Virus For about 40 years before that, much of medicine was convinced a bacterium was to blame. That 1933 discovery is the starting point for every flu shot and flu forecast since.
How Three Scientists Caught the Flu Virus in 1933
In 1933 a flu epidemic swept England, and Wilson Smith, Christopher Andrewes and Patrick Laidlaw collected samples from infected people.2CDC Museum. Discovering the Virus They filtered the samples and tried to infect many kinds of animals, looking for any that would show flu symptoms.3CDC Museum. Discovering the Virus
Many common lab animals simply don’t get sick from human flu, so the team had to search. Ferrets turned out to be the answer. They sneeze, run fevers and get runny noses much like people do, so infection is easy to spot, and researchers still use them for the same reason. The first attempt, in early February, failed when a distemper outbreak wiped out the ferrets before the virus could be isolated.4the Guardian. 80 years ago today: MRC researchers discover viral cause of flu
Then Smith himself caught flu on 4 March, and Andrewes used Smith’s throat garglings to infect fresh ferrets.5the Guardian. 80 years ago today: MRC researchers discover viral cause of flu By the third day two ferrets showed clear flu symptoms, the first proof that the human flu agent could be passed to an animal.6CDC Museum. Discovering the Virus
The agent passed through a filter fine enough to stop bacteria, could not be seen under a microscope, and would not grow in a lab dish, yet it still made animals sick.7the Guardian. 80 years ago today: MRC researchers discover viral cause of flu A filter with tiny pores lets fluid through but traps bacteria and larger cells, so if the filtered fluid still causes illness, the cause must be smaller than bacteria.
The Bacterium Everyone Blamed First
In 1892, during a flu pandemic, German scientist Richard Pfeiffer isolated a small rod-shaped bacterium from the noses of flu patients.8PubMed Central. Influenza: exposing the true killer He announced in 1892 and 1893 that he had found flu’s cause and named it Bacillus influenzae.9PubMed Central. The state of science, microbiology, and vaccines circa 1918 The bacterium was a major focus of research and argument from 1892 until about 1920.10PubMed Central. The state of science, microbiology, and vaccines circa 1918 So during the 1918 pandemic, many doctors were still hunting the wrong germ.
The bacterium is real and still carries the name Haemophilus influenzae, even though it doesn’t cause flu.11The Journal of Immunology. The 1918–1919 Influenza Pandemic as Covered in The JI The Hib vaccine children get protects against Haemophilus influenzae, not flu. You still need a flu vaccine.
The mistake made sense, since bacteria often infect the lungs of people already weakened by flu, so they turn up in sick patients.12PubMed Central. Two complete 1918 influenza A/H1N1 pandemic virus genomes characterized by next-generation sequencing using RNA isolated from formalin-fixed, paraffin-embedded autopsy lung tissue samples along with evidence of secondary bacterial co-infection A germ found in sick people may be a bystander or a secondary invader taking advantage of weakened airways, and finding it is not the same as proving it caused the illness. To prove that, you have to show that the germ alone, taken from a patient, can reproduce the illness in a healthy host. Pfeiffer’s bacterium failed that kind of test, and the 1933 ferret experiment passed it using a filtered fluid with no bacteria in it.
Pigs and Birds Came First
American scientist Richard Shope discovered a flu virus in pigs in 1931, two years before the human virus was isolated.13The Lancet. Influenza Infection of Man from the Ferret Shope’s pig work showed that a flu-like illness could be traced to a virus rather than a bacterium, giving researchers good reason to suspect human flu was viral too. Around 1900 scientists had shown that “fowl plague” in poultry was caused by a virus.14Centers for Disease Control and Prevention. Highlights in the History of Avian Influenza But it was not until 1955 that this bird virus was shown to be a type A flu virus, what we now call bird flu.15Centers for Disease Control and Prevention. Highlights in the History of Avian Influenza So depending on how you count, the first flu virus found was the bird one, the pig one, or the human one.
1933 is the standard answer because it’s the first flu virus found in people, and quiz answers and textbooks usually mean the human virus. 1931 is the first virus recognised as flu, found in pigs. The fowl plague virus was identified first, but no one knew it was flu for decades.
Flu B Joins the Picture in 1940
In 1940 American scientist Thomas Francis Jr. isolated influenza B, which had been hard to tell apart from influenza A.16PubMed Central. Dr. Thomas Francis Jr.: A Legacy of Vaccine Innovation and Public Health Advancement What gave it away was blood: serum from recovered people and animals reacted to the new virus differently than to the type found in 1933.17Experimental Biology and Medicine. Differentiation of Influenza A and Influenza B by the Complement-Fixation Reaction The different reactions showed two distinct viruses had been mixed together under one name. The 1933 virus became known as flu A and the new one as flu B, which paved the way for vaccines that protect against both types.18PubMed Central. Dr. Thomas Francis Jr.: A Legacy of Vaccine Innovation and Public Health Advancement
Both types cause seasonal flu with similar symptoms. Flu A is the type found in birds and pigs and behind pandemics, while flu B mostly circulates among people. Yearly flu shots still include protection against both.
Seeing and Mapping the Virus
In 1941 George Hirst noticed that flu virus made red blood cells clump together.19PubMed Central. Etymologia: Hemagglutinin and Neuraminidase This led to the discovery of hemagglutinin, the H protein on the virus’s surface.20PubMed Central. Etymologia: Hemagglutinin and Neuraminidase The H protein lets the virus latch onto a cell to start an infection.21PubMed Central. Influenza Hemagglutinin and Neuraminidase: Yin–Yang Proteins Coevolving to Thwart Immunity Its partner, neuraminidase, the N protein, cuts the virus loose so new copies can escape and spread.22PubMed Central. Influenza Hemagglutinin and Neuraminidase: Yin–Yang Proteins Coevolving to Thwart Immunity
Hirst’s observation also gave scientists a simple lab test: mix flu virus with red blood cells and watch whether they clump. The test could detect and roughly measure virus in a sample, and because antibodies block the clumping, it could also show whether a person’s blood had defences against a strain.
Names like H1N1 describe which versions of the two proteins a flu strain carries. The numbers mark the versions, and both the 1918 and 2009 pandemics were H1N1 strains. Bird flu headlines often name other combinations, like H5N1.
The N protein matters for treatment too. Common flu medicines work by blocking it, so if new copies can’t escape the cell, the infection spreads more slowly. That is why these drugs work best when started early.
From Discovery to the First Flu Shot
In the 1940s Thomas Francis Jr. and Jonas Salk led University of Michigan work on the first inactivated flu vaccine, with US Army support.23World Health Organization. History of influenza vaccination They found that flu virus made harmless with diluted formaldehyde still triggered an immune response as strong as live virus.24University of Michigan Heritage Project. The First Flu Shot Less than a year after starting, Francis told the War Department he had a vaccine ready to test.25University of Michigan Heritage Project. The First Flu Shot It was tested on US military personnel and licensed for wider use in 1945.26World Health Organization. History of influenza vaccination Barely 12 years had passed from finding the virus to a licensed vaccine.
Formaldehyde stops the virus from replicating while leaving its outer proteins recognisable. After vaccination, the body makes antibodies against them and keeps a memory, so it can respond faster if the real virus arrives. Many flu shots today still use this basic idea, and the virus in them is killed and cannot cause flu.
Why Knowing the Virus Hasn’t Ended Flu
Flu viruses keep changing their surface proteins through processes called antigenic drift and shift.27PubMed. Influenza Virus: Dealing with a Drifting and Shifting Pathogen With drift, small changes build up until your existing antibodies struggle to recognise the virus.28CDC. How Flu Viruses Can Change: Drift and Shift
Shift is a bigger jump, when a flu A virus with new H or N combinations from animals reaches people.29CDC. How Flu Viruses Can Change: Drift and Shift Most people then have little or no immunity, as happened with the 2009 H1N1 pandemic, whose genes came from pigs, humans and birds.30CDC. How Flu Viruses Can Change: Drift and Shift The animal viruses found in 1931 and earlier are the raw material for shift. Pigs can carry both bird and human flu, letting genes mix, and watching animal flu is how scientists try to spot the next pandemic early.
Because of all this, flu vaccines must be updated often to match the strains going around.31PubMed. Influenza Virus: Dealing with a Drifting and Shifting Pathogen Getting vaccinated each season keeps protection current.
Bringing the 1918 Virus Back to Study It
Nobody could isolate the 1918 virus at the time, since the human flu virus wasn’t found until 1933.32CDC Museum. Discovering the Virus But in 1996 scientists began recovering damaged fragments of its genetic material from preserved tissue of 1918 victims.33PubMed Central. Reconstruction of the 1918 Influenza Virus: Unexpected Rewards from the Past These pieces eventually let researchers rebuild the complete 1918 virus.34PubMed Central. Reconstruction of the 1918 Influenza Virus: Unexpected Rewards from the Past Almost a century later, it yielded clues about why flu turns deadly and how to prepare for future pandemics.35PubMed Central. Reconstruction of the 1918 Influenza Virus: Unexpected Rewards from the Past
Later work on preserved lung samples from US army hospitals raised the number of complete 1918 virus genomes from four to six.36PubMed Central. Two complete 1918 influenza A/H1N1 pandemic virus genomes characterized by next-generation sequencing using RNA isolated from formalin-fixed, paraffin-embedded autopsy lung tissue samples along with evidence of secondary bacterial co-infection Those samples showed signs of bacterial infections alongside the virus, which helps explain Pfeiffer’s error.37PubMed Central. Two complete 1918 influenza A/H1N1 pandemic virus genomes characterized by next-generation sequencing using RNA isolated from formalin-fixed, paraffin-embedded autopsy lung tissue samples along with evidence of secondary bacterial co-infection