99 Fun Facts About Influenza

How long after exposure do influenza symptoms typically begin?

About two days, within a range of one to four. Flu then tends to strike abruptly, where a cold usually creeps up with a runny or stuffy nose.

Which virus, unrelated to influenza, is the leading cause of the vomiting bug nicknamed 'stomach flu'?

Norovirus. True flu is an infection of the airways, and vomiting is uncommon in adults who catch it, though children with flu do sometimes get it.

Which animals are the primary reservoir of influenza A viruses?

Aquatic birds. Wild ducks can carry the virus in their gut without getting sick, and with a reservoir like that on the wing, the WHO calls eradicating flu impossible.

In 2024, H5N1 bird flu jumped species and began spreading through which US farm herds?

Cattle. The virus passed between dairy herds in several states, and some farm workers caught it from the cows, mostly with mild symptoms; tests showed pasteurisation kills it in milk.

In the subtype name H1N1, what does the H stand for?

Haemagglutinin. The protein is named for a lab quirk, its knack of making red blood cells clump together; on the virus it is the spike that latches onto the cells of the airway.

As of 2025, which two influenza A subtypes circulate in people as seasonal flu?

H1N1 and H3N2. Today's H1N1 is the 2009 pandemic strain, which pushed the older seasonal H1N1 out of circulation and then settled in as an ordinary winter virus.

Flu subtypes H17 and H18 were first found in which Latin American animals?

Bats. CDC researchers sampling little yellow-shouldered bats in Guatemala in 2009-10 found a flu virus unlike any seen before; only 16 H types had been known until then.

Influenza B has no subtypes but two lineages named after which two places?

Victoria and Yamagata. No Yamagata virus has been confirmed anywhere in the world since March 2020, so American flu shots dropped that lineage from the 2024-25 season.

Which influenza type causes seasonal epidemics but has never caused a pandemic?

Influenza B. It infects almost nothing but people, so it seems to lack the animal reservoir from which type A keeps drawing strains that no human immune system has met.

What kind of genome do influenza viruses have?

Segmented negative-sense RNA. The genome comes in separate pieces, eight of them in types A and B, so two strains that infect the same cell can trade whole genes.

The 1918 flu virus was rebuilt in 2005 using genes recovered from a victim buried where?

In Alaskan permafrost. Johan Hultin dug at the mass grave in Brevig Mission as a student in 1951 and found no live virus; he went back in 1997 and took frozen lung tissue that still held its genes.

What is the gradual build-up of mutations that changes a flu virus's antigens called?

Antigenic drift. Small copying errors pile up until the antibodies from an earlier infection or shot no longer recognise the virus, which is why a person can catch flu many times in a lifetime.

In which US city did an 'Anti-Mask League' form in 1919 to fight a flu mask law?

San Francisco. The city had fined or jailed people caught barefaced in 1918, and when masks were ordered back the next January a league that counted doctors and a city supervisor among its members rallied against them.

Which farm animals are called flu 'mixing vessels' because they can catch both bird and human strains?

Pigs. When a bird strain and a human strain infect the same pig they can swap genes and emerge as a new hybrid; the 2009 pandemic virus was a blend of swine, bird and human flu genes.

Which cell-surface molecule do influenza HA proteins bind to?

Sialic acid. Human strains grip a form of it that lines the nose and throat, while bird strains prefer a form found deep in human lungs, which may be why bird flu passes so poorly between people.

Which common painkiller should children with flu avoid because of its link to Reye's syndrome?

Aspirin. Once parents in America and Britain were warned off giving it to feverish children in the 1980s, Reye's syndrome, a sudden swelling of the liver and brain, became very rare.

Which two bacteria most often cause secondary pneumonia after flu?

Streptococcus pneumoniae and Staphylococcus aureus. Follow-on infections like these, rather than the virus alone, are thought to have caused most deaths in the 1918 pandemic, before antibiotics existed.

The Liberty Loan parade of 28 September 1918, blamed for a huge flu outbreak, was held in which US city?

Philadelphia. Some 200,000 people packed the route although flu had already reached the city; within days its hospital beds were full, and its death rate ended among the worst in the nation.

Up to how many respiratory deaths does the WHO estimate seasonal flu causes worldwide each year?

650,000 is the top of the WHO's range, which starts at 290,000. That is more than the CDC's highest estimate of deaths in the first year of the 2009 flu pandemic.

Which age group is considered the most efficient spreader of influenza?

Schoolchildren. They catch flu easily and carry it home, which is why Britain vaccinates schoolchildren partly to shield the babies and grandparents they live with.

When does a person with flu typically start being infectious?

A day before symptoms. People shed the virus before they feel ill and for five to seven days afterwards, so staying home once you feel sick comes too late to stop every infection.

Most flu vaccines are manufactured by growing the virus in what?

Embryonated chicken eggs. In a method more than 70 years old, the virus is injected into fertilised hens' eggs, left to multiply for several days, then harvested from the fluid and killed.

How often does the WHO meet to choose flu vaccine strains?

Twice a year, once per hemisphere. Experts meet in February to pick strains for the northern winter and in September for the southern one, because making the vaccine takes about six months.

How are live attenuated influenza vaccines given?

Sprayed into the nose. The vaccine's weakened viruses are bred to multiply only at the cooler temperature of the nose, so they stir up immunity there without taking hold in the warmer lungs.

Which US president ordered a mass swine flu vaccination drive for a pandemic that never came?

Gerald Ford. In 1976 one recruit's death at an army base set off a drive that vaccinated more than 40 million Americans in under three months, until reports of a rare paralysis, Guillain-Barré syndrome, halted it.

Oseltamivir works by blocking which flu protein, the N in subtype names such as H1N1?

Neuraminidase. The enzyme cuts newly made viruses loose from the infected cell; with it blocked they stay stuck to the surface instead of spreading to fresh cells.

Under what brand name is oseltamivir sold?

Tamiflu. Governments stockpiled it in 2005 against a feared bird flu pandemic, yet a 2014 Cochrane review found that it eased adults' symptoms only about 17 hours sooner.

Oseltamivir is made from shikimic acid, originally extracted from which plant?

Chinese star anise. The spice yields so little of the acid that the drug ran short when governments rushed to stockpile it in 2005, and makers turned to engineered E. coli bacteria to brew it instead.

Which flu antiviral from the 1960s later found a second use treating Parkinson's disease?

Amantadine. The second use outlived the first: flu viruses grew so resistant to the drug that it is no longer recommended against them, while Parkinson's patients still take it.

Before the flu virus was found in 1933, doctors had long blamed influenza on what kind of organism?

A bacterium. Richard Pfeiffer found it in the sputum of flu patients and blamed it for the disease; the mistake survives in its name, Haemophilus influenzae, though what it can cause is meningitis.

British scientists first isolated the human flu virus in 1933 by infecting which animals?

Ferrets. At a London laboratory, Wilson Smith, Christopher Andrewes and Patrick Laidlaw gave the animals flu with throat washings from patients, the first proof that a virus was the cause.

Which polio-vaccine pioneer helped develop the first US flu vaccine?

Jonas Salk. He worked under Thomas Francis on a killed-virus flu vaccine that the Army, mindful of its losses to flu in the First World War, tested on its own troops; civilians got it in 1945.

Which artist painted 'Self-Portrait with the Spanish Flu' in 1919?

Edvard Munch. He fell ill around New Year 1919, possibly with the pandemic flu, and recorded the illness stage by stage in sketches and paintings; the gaunt, open-mouthed face echoes his own Scream.

Despite its name, the 1918 'Spanish flu' had its earliest documented cases in which country?

The United States. Soldiers at a Kansas army camp fell ill in March 1918; the 'Spanish' label stuck because neutral Spain's uncensored newspapers reported the epidemic while the warring nations hushed it up.

Which subtype caused the 1957 Asian flu pandemic?

H2N2. Eleven years later it picked up a new H gene from a bird virus and came back as H3N2, the strain behind the 1968 pandemic, still carrying the N2 it had in 1957.

Which American microbiologist spotted the coming 1957 flu pandemic and got 40 million vaccine doses made within months?

Maurice Hilleman. He went on to create more than 40 vaccines; his mumps vaccine was grown from a swab of his own five-year-old daughter's throat and still carries her name, Jeryl Lynn.

Who carried the 1968 Hong Kong flu across the Pacific to California?

Soldiers back from Vietnam. The new virus had infected some 500,000 people in Hong Kong that summer and spread through the region before returning troops brought it home with them.

How is the 1977 reappearance of H1N1 thought to have happened?

Release from a laboratory freezer is one leading explanation: the 1977 virus was nearly identical to strains from the 1950s, as if it had not evolved since, and it sickened mostly people under 26.

The earliest known case of the 2009 H1N1 pandemic was traced to a small boy in which place?

La Gloria, Mexico. The boy fell ill in the Veracruz village in March 2009 and recovered; the new virus itself was only identified weeks later, by laboratories in America and Canada.

Unlike seasonal flu, the 2009 H1N1 pandemic hit which group far less than expected?

Adults over 60. Nearly a third of them carried antibodies from an older H1N1 met earlier in life, so four in five deaths were among people under 65, the reverse of an ordinary flu season.

Highly pathogenic H5N1 was first detected in 1996 in which animals?

Geese in Guangdong. The virus drew little notice until it reached Hong Kong's poultry markets in 1997 and killed 6 of 18 infected people; the territory ended that outbreak by slaughtering all its poultry.

By the classic definition, what makes a bird flu strain 'highly pathogenic'?

75% or more of infected chickens die. The label describes only what a strain does to chickens in the laboratory, and does not predict how dangerous the same virus would be to people.

Which avian subtype began infecting people in China in 2013, starting in Shanghai?

H7N9. It caused more than 1,500 reported human cases and many deaths between 2013 and 2019, yet for its first few years it was only mildly harmful to the poultry that carried it.

The name influenza comes from an old belief that the disease was caused by the influence of what?

The stars. The word first meant an astrological 'flowing in' of ethereal fluid from the heavens; by about 1500 it was being used for outbreaks of disease, which were put down to that unseen influence.

The word influenza reached English in 1743, during an epidemic sweeping Europe, from which language?

Italian. In April 1743 The Gentleman's Magazine told its readers of a report from Rome that 'a sort of plague' had broken out there, 'and they call it the Influenza'.

What is the French-derived alternative name for influenza?

La grippe. English borrowed the word in the 1770s; it probably comes from the French gripper, to seize, supposedly for the way the illness clutches at the throat.

The Great Epizootic of 1872, which brought transport in North American cities to a halt, was flu among which animals?

Horses. The outbreak began in Canada that autumn and laid up most city horses for a week or two; when fire swept Boston in November, teams of volunteers had to drag the fire wagons by hand.

What is the exaggerated immune response that can accompany severe flu pneumonia called?

Cytokine storm. The immune system floods the lungs with inflammatory signals and damages them itself; one theory is that this is why the 1918 flu killed so many healthy young adults, whose defences were strongest.

Which Pacific territory kept the 1918 flu out completely with a strict naval quarantine?

American Samoa. The US Navy sealed the islands off and no flu deaths were reported there, while in Western Samoa, where the virus arrived from Auckland, more than a fifth of the people died.

Which NBA star's 38-point night against Utah in the 1997 Finals is remembered as 'The Flu Game'?

Michael Jordan played 44 minutes of Game 5 while visibly ill, then collapsed into a teammate's arms. He later blamed food poisoning from a pizza rather than flu.

Which sport's 1919 final was abandoned with no winner after flu hospitalised most of one team?

Ice hockey's 1919 Stanley Cup Final between Montreal and Seattle was called off after five games, and Canadiens defenceman Joe Hall died four days later.

Which US president's German-born paternal grandfather died in New York during the 1918 flu pandemic?

Donald Trump's grandfather Frederick was a barber and restaurant-hotel manager who had just begun buying real estate in Queens when he died in May 1918.

Doctors in 1918 dreaded seeing a flu patient's face turn which colour, spreading out from the cheekbones?

Blue faces, a sign doctors called heliotrope cyanosis, marked the gravest cases. The blue-violet tinge of dying patients gave the illness the nickname 'purple death'.

Which Twilight character was dying of Spanish flu in Chicago in 1918 when he was turned into a vampire?

Edward Cullen was transformed by Carlisle, a doctor, after Edward's dying mother begged him to do whatever it took to save her son.

Which Stephen King novel opens with a weaponised flu called Captain Trips escaping a secret desert lab?

The Stand's superflu wipes out about 99.4% of the world's population within a month. King later restored more than 300 cut pages for a 1990 uncut edition.

Which country, then under British rule, lost an estimated 12 million or more people to the 1918 flu?

India lost about 5% of its people, and 1911 to 1921 became the only census decade in which its population fell.

Which tech company's Flu Trends tool, launched in 2008, tried to track outbreaks from people's web searches?

Google stopped publishing Flu Trends estimates in 2015 after the tool badly overshot, predicting twice as many doctors' visits as the CDC recorded in one stretch of the 2012 season.

Unlike most flu outbreaks, the 1918 pandemic was unusually deadly for healthy people in which age group?

Deaths at ages 20 to 40 made up nearly half of the US pandemic toll, and 99% of the Americans who died were under 65.

At a zoo in Thailand, an H5N1 bird flu outbreak killed more than 140 of which big cats?

Tigers died in such numbers that health officials culled every sick animal to stop the zoo becoming a reservoir for the virus.

According to the CDC, which symptom are people with a common cold more likely to have than people with flu?

A runny or stuffy nose points towards a cold, while flu symptoms are typically more intense and begin more abruptly.

Which pattern of fever or cough does the CDC list as an emergency warning sign of flu complications?

When a fever or cough improves, then returns or worsens, the CDC says to get medical care right away; for babies under 12 weeks, any fever at all is on the same list.

Which pair of milder illnesses does the CDC give as examples of moderate complications of flu?

Sinus and ear infections sit at the milder end of the CDC's list, while pneumonia is the serious complication. Most people recover in a few days to under two weeks.

Among the possible serious complications of flu is myocarditis, an inflammation of which organ?

The heart is not the only target: the CDC also lists inflammation of the brain and of muscle tissue among the serious complications flu can trigger.

According to the CDC, a change in or loss of which sense is more frequent with COVID-19 than with flu?

Smell, along with taste, can change or vanish in either illness, but the CDC notes that it happens more often with COVID-19.

According to the CDC, which is the most common medical condition among children hospitalized with flu?

Asthma leaves the airways swollen and sensitive, so flu can inflame them further and set off attacks. People with it are also more likely to develop pneumonia after flu.

The US flu season runs from October to May and typically peaks in which month?

February is the usual peak of the American season, while Australia's, which runs from May to October, tends to peak in August.

Experiments on lab animals showed flu viruses spread through the air best when it is cold and what else?

Dry air boosts the airborne spread of flu, one reason epidemics in temperate countries peak in winter, when humidity and sunlight are both low.

The 2007 US Pandemic Severity Index graded flu pandemics from 1 to 5, echoing the scale for which hazard?

Hurricanes gave the index its five categories, with Category 5 reserved for a worst case on the scale of 1918. A different framework replaced it in 2014.

Which lab technique, the gold standard for diagnosing flu, can also tell influenza A subtypes apart?

RT-PCR, short for reverse transcription polymerase chain reaction, detects the genetic material of the virus, though it costs more than a rapid test.

Which season of 1918 saw the flu pandemic's death rate peak, after ships carried a new wave to Boston?

Autumn brought a wave much deadlier than the mild one before it, and October 1918 had the highest death rate of the whole pandemic.

Which US president fell ill at the 1919 Paris Peace Conference, in what some experts believe was Spanish flu?

Woodrow Wilson spent six months in Europe negotiating the peace as one of the 'Big Four' Allied leaders, pressing above all for a League of Nations.

Which Austrian Expressionist died of Spanish flu in Vienna in 1918, days after his pregnant wife Edith?

Egon Schiele was 28 and outlived Edith by three days. For fear of contagion, visitors spoke to him by way of a mirror set on the threshold of his room.

Which poet, wounded fighting for France in World War I, died of Spanish flu two days before the Armistice?

Guillaume Apollinaire, who coined the term Surrealism in 1917, died at 38 and was officially recognised as having fallen for France.

Which Spanish king's freely reported illness in 1918 helped saddle his neutral country with the pandemic's name?

Alfonso XIII recovered, but his uncensored press covered the illness while the warring nations hid their own outbreaks, so Spain looked like the worst-hit place.

Japan nicknamed the 1918 flu after athletes of which sport, blamed for bringing it home from Taiwan?

Sumo wrestlers took the blame for carrying the disease back, so it became 'sumo flu' in Japan, while Brazilians called the same pandemic the 'German flu'.

Which 1889 pandemic, long assumed to be influenza, do some researchers now suspect was caused by a coronavirus?

The Russian flu raced around the world along new railways and steamship routes, and several authors in the early 2020s proposed human coronavirus OC43 as its real cause.

Which British prince, in direct line to the throne and newly engaged, died in a flu pandemic in 1892?

Prince Albert Victor died less than a week after his 28th birthday, and his fiancée Mary of Teck later married his younger brother, the future George V.

Under which older name was highly pathogenic bird flu first identified in 1878 by Edoardo Perroncito?

Fowl plague was told apart from other deadly poultry diseases in 1878, but it took until 1955 to show its virus was a close relative of human influenza.

Unlike most viruses of its kind, influenza copies its genetic material in which part of the host cell?

The nucleus is an unusual home for an RNA virus, since most copy themselves in the cytoplasm. Flu goes there to steal the caps from the cell's own messages, a trick called cap snatching.

In the early 1940s George Hirst found that flu viruses make what clump together, a trick labs still use?

Red blood cells link up with virus particles into a lattice, which gave Hirst a simple way to measure how much virus a sample contains.

Which immune protein did Alick Isaacs and Jean Lindenmann discover in 1957 while experimenting with flu virus?

Interferon takes its name from viral interference: membranes exposed to heat-killed flu virus released a protein that stopped live virus from growing.

Influenza viruses belong to which family, whose name comes partly from the Greek word for mucus?

Orthomyxoviridae contains nine genera, but only four of them hold the viruses that cause flu in birds and mammals.

Dog flu was first recognised in 2004 after a deadly respiratory outbreak among which breed in Florida?

Greyhounds at a Florida racetrack were the first known canine victims of the H3N8 virus, and a different dog flu strain, H3N2, turned up in the US in 2015.

A 2022 H5N1 outbreak on a farm in Spain alarmed scientists by apparently spreading between which fur animals?

Mink on the farm gave the first recorded evidence of H5N1 passing from mammal to mammal, and about 4 percent of the animals died.

Which flu antiviral, approved in Japan and the US in 2018, treats the illness with a single dose by mouth?

Baloxavir, sold as Xofluza, blocks the 'cap snatching' step that the virus's polymerase depends on, and it can also be given to prevent flu after exposure.

In the official name of a flu virus isolate, what does the place written after the type letter record?

Where it was collected is only part of the label: the name also carries a lab reference number and the year, and animal viruses add the host species.

Which American identified the virus behind swine flu in 1931, two years before the human one was found?

Richard Shope worked at the Rockefeller Institute, and he later showed that people who had lived through 1918 still carried antibodies against the swine virus.

In which Emily St. John Mandel novel does the fictional 'Georgia Flu' wipe out most of the world's population?

Station Eleven is set around the Great Lakes before and after the collapse, and its flu kills 99% of the people it infects within days.

British troops in spring 1918 named the new flu after which Belgian battlefield region where many fell ill?

Flanders gave both sides a name for the illness: German troops spoke of 'Flandern-Fieber', or Flemish fever.

In Downton Abbey, which fiancée of Matthew Crawley dies of Spanish flu in series two?

Lavinia Swire, played by Zoe Boyle, dies in 1919, and Matthew struggles to move past it before he and Mary finally wed in 1920.

Which WHO Director-General was accused of 'crying wolf' after the 2009 flu pandemic proved milder than feared?

Margaret Chan had earlier run Hong Kong's health department during its 1997 H5N1 bird flu outbreak, and she led the WHO from 2006 to 2017.

Which American virologist, with Ann Reid and Johan Hultin, first sequenced the genome of the 1918 flu virus?

Jeffery Taubenberger's team worked from preserved tissue of victims, among them an army private who died at a South Carolina camp in September 1918.

Two of the three child visionaries of which Marian shrine town died as victims of the 1918 flu epidemic?

Fátima's young brother and sister, the Marto siblings, died in 1919 and 1920, and both were canonised at the shrine in 2017.

Which British co-author of a secret 1916 pact dividing the Ottoman Middle East died of Spanish flu in Paris?

Mark Sykes was buried in a lead-lined coffin, and in 2008 his remains were exhumed in the hope that it had preserved the virus for research.

The 1918 flu pandemic more than tripled the sales of which cold remedy in just one year?

VapoRub sales leapt from $900,000 to $2.9 million in a single year as the epidemic spread across America.

The CDC describes flu as an illness caused by viruses that infect the nose, the throat and which other organs?

The lungs are where flu turns serious, since the virus alone can be enough to cause pneumonia.

Which European country, newly independent in 1918, nicknamed that year's pandemic flu the 'Bolshevik disease'?

Poland's label was one of many that pinned the pandemic on outsiders; in Senegal, for instance, people called it the 'Brazilian flu'.

Which Australian state stayed free of the pandemic flu until mid-August 1919, longer than any other?

Tasmania held out longest after a strict maritime quarantine had shielded the whole country through 1918; the flu only landed on the mainland in January 1919.

Which rare paralysing nerve disorder was linked to the vaccine in America's 1976 swine flu drive?

Guillain–Barré syndrome reports rose once the programme began; the whole drive had been triggered by an outbreak at Fort Dix, New Jersey, that caused one death.