What is the Influenza Virus? 

Understanding the Origin, Spread, and Evolution of the Influenza Virus-Centre for New Immigrant Well-Being (CNIW)

Influenza, usually called the flu, is a contagious illness that affects the lungs and breathing. It spreads through small droplets that are released when someone who is sick coughs, sneezes, or talks, or by touching things that have the virus on them. People can spread the flu even before they show symptoms; this is called asymptomatic transmission.1

Common flu symptoms include fever, chills, sore throat, feeling very tired, muscle aches, and cough. Most people recover from the flu within about two weeks, but sometimes it can lead to serious problems like pneumonia, or even death in severe cases.2 People who are most at risk for severe illness include young children, older adults, pregnant people, and those who already have health issues, such as asthma, diabetes, or heart disease.1 

Different Types of Influenza Virus

There are 4 main types of influenza viruses:

  • Influenza A is the most common type. It causes seasonal flu epidemics each year and has caused all known flu pandemics (worldwide outbreaks). It can infect animals and people, and usually makes people sicker than other flu types. Flu type A is divided into subtypes based on certain proteins found on the virus surface (like H1N1 or H3N2).11,12
  • Influenza B also causes seasonal flu epidemics, but it does not cause pandemics. It only infects people, changes more slowly, and usually causes milder illness than type A.12,13
  • Influenza C usually only causes mild symptoms, and isn't considered a major health concern.11,12
  • Influenza D mostly affects cattle (cows) and does not cause illness in people.11

Every flu season, different versions of influenza A and B circulate. This is why the flu vaccine needs to be updated every year.11

Origin of the Influenza Virus

Influenza viruses probably first came from bird viruses thousands of years ago. Wild birds, such as ducks and gulls, naturally carry many types of influenza A viruses without getting sick. This lets the viruses stay around in nature.3

Sometimes these bird flu viruses can move from birds to other animals, like pigs or humans. Pigs play an important role as “mixing vessels” because they can get infected by both bird and human flu viruses at the same time. When this happens, viruses can mix their genetic material inside pigs, creating new flu strains that can infect humans.4 This mixing is thought to be an important reason why new influenza viruses sometimes appear and cause big human outbreaks.5

Understanding the Origin, Spread, and Evolution of the Influenza Virus-Centre for New Immigrant Well-Being (CNIW)

Figure 1. How influenza A viruses move between different animal species and people.6

How the Virus Mutates Over Time

Influenza viruses change quickly, making it hard for our bodies to keep up. Because the virus changes so often, new flu vaccines must be created regularly to protect people.7

2 main processes make these changes happen:

  • Antigenic drift means small changes that happen slowly over time in the virus’s genetic material. Over months or years, these small changes can add up and create new strains of the virus. These new strains can avoid immunity that people got from past infections or vaccinations.8
  • Antigenic shift means a sudden, big change in the flu virus. This can create a totally new type of flu that most people have no protection against. This greatly raises the risk of a worldwide pandemic.8

Because flu viruses change all the time, groups like the Centers for Disease Control and Prevention (CDC) in the U.S., the Public Health Agency of Canada (PHAC), and the World Health Organization (WHO) keep a close watch on flu viruses around the world. They test and study samples year-round to decide what viruses should be included in each year’s flu vaccine.9,10 

Major Pandemics in History

Throughout history, influenza has caused several huge pandemics. These pandemics made many people around the world very sick, overwhelmed hospitals, and disrupted daily life and economies.14 Each pandemic started with a new flu type that people hadn’t been exposed to before, which allowed it to spread rapidly.15

The worst pandemic was the 1918 Spanish flu, caused by an H1N1 influenza A virus, likely from birds.16 It infected about one-third of all people on earth at the time and killed over 50 million people, making it one of the deadliest pandemics in recorded history.16 Usually, seasonal flu mostly affects young children and older adults, but the 1918 flu strangely caused many deaths in young, healthy adults as well.16 It was named the “Spanish flu” not because it started in Spain, but because Spain openly reported about it in the news during World War I, while other countries did not.17

Understanding the Origin, Spread, and Evolution of the Influenza Virus-Centre for New Immigrant Well-Being (CNIW)

Figure 2. One of the first outbreaks of the 1918 influenza pandemic in the U.S. started in a World War I military training camp in Kansas.18

The most recent flu pandemic happened in 2009, and it was called the "swine flu." This was also caused by the H1N1 influenza A virus. It first appeared in Mexico and the United States and had genetic material from pigs, birds, and people.19 Children and younger adults were most affected, but there were fewer deaths compared to past pandemics. This was partly due to a quick international response, new antiviral medicines, and older adults already having some protection because they had been exposed to similar viruses earlier in their lives.19,20,21

Prevalence of Seasonal Influenza

Pandemics get a lot of attention, but seasonal flu still causes a lot of illness every year. Every year, flu infects up to 1 billion people worldwide. It causes between 3 and 5 million serious illnesses and up to 650,000 deaths each year.11 In Canada, flu season usually peaks between November and April.22 Each year, about 5-10% of adults and 20-30% of children get the flu, resulting in thousands of hospital stays.23

How Can I Avoid Getting Sick?

Getting a flu shot every year is the best way to protect yourself from getting sick. You can also lower your risk by washing your hands often, covering your mouth and nose when coughing or sneezing, staying home when you feel sick, avoiding close contact with others who are sick, and not touching your eyes, nose, or mouth with unwashed hands.24

Why Influenza Still Matters Today

Even though there have been many advances in vaccine development, worldwide tracking, and public health education, influenza remains a serious health threat. Because the virus constantly changes, it never fully goes away and keeps coming back each year in new forms. Yearly flu outbreaks still put a lot of pressure on people, families, and healthcare systems. This ongoing public health issue shows how important it is to get vaccinated each year and stay alert to protect yourself, your family, and your community from one of the oldest and most stubborn infectious diseases in the world.

References

  1. World Health Organization. (2025). History of influenza vaccination. https://www.who.int/news-room/spotlight/history-of-vaccination/history-of-influenza-vaccination. Accessed July 9, 2025.
  1. Centers for Disease Control and Prevention. (2024). Signs and Symptoms of Flu. https://www.cdc.gov/flu/signs-symptoms/index.html. Accessed July 9, 2025.
  1. Willie, M. and Holmes, E. C. (2020). The Ecology and Evolution of Influenza Viruses. Cold Spring Harb Perspect Med. 10, a038489. doi: 10.1101/cshperspect.a038489
  1. Abdelwhab, E. M. and Mettenleiter, T. C. (2023). Zoonotic Animal Influenza Virus and Potential Mixing Vessel Hosts. Viruses. 15, 980. doi: 10.3390/v15040980
  1. Medina, R. A. and García-Sastre, A. (2023). Influenza A viruses: new research developments. Nat Rev Microbiol. 9, 590–603. doi: 10.1038/nrmicro2613
  1. Shi, Y., Wu, Y., Zhang, W., Qi, J. and Gao, G. F. (2014). Enabling the 'host jump': structural determinants of receptor-binding specificity in influenza A viruses. Nat Rev Microbiol. 12, 822–831. doi: 10.1038/nrmicro3362
  1. Gupta, D. and Mohan, S. (2023). Influenza vaccine: a review on current scenario and future prospects. J Genet Eng Biotechnol. 21, 154. doi: 10.1186/s43141-023-00581-y
  1. Centers for Disease Control and Prevention. (2024). How Flu Viruses Can Change: "Drift" and "Shift". https://www.cdc.gov/flu/php/viruses/change.html. Accessed July 9, 2025.
  1. Government of Canada. (2019). Flu Fighters: How Canada influences the development of the annual flu shot. https://science.gc.ca/site/science/en/blogs/science-health/flu-fighters-how-canada-influences-development-annual-flu-shot. Accessed July 9, 2025.
  1. Centers for Disease Control and Prevention. (2024). CDC's World Health Organization (WHO) Collaborating Center for Surveillance, Epidemiology and Control of Influenza. https://www.cdc.gov/flu/php/who-collaboration/index.html. Accessed July 9, 2025.
  1. World Health Organization. Influenza (seasonal). (2025). https://www.who.int/news-room/fact-sheets/detail/influenza-(seasonal). Accessed July 9, 2025.
  1. Gatta, F. (2025). Types of Flu. https://www.webmd.com/cold-and-flu/advanced-reading-types-of-flu-viruses. Accessed July 9, 2025.
  1. Healthline Media. (2025). Influenza A vs. B: What to know. https://www.medicalnewstoday.com/articles/327397. Accessed July 9, 2025.
  1. Saunders-Hastings, P. R. and Krewski, D. (2016). Reviewing the History of Pandemic Influenza: Understanding Patterns of Emergence and Transmission. Pathogens. 5, 66. doi: 10.3390/pathogens5040066
  1. Centers for Disease Control and Prevention. (2024). Types of Influenza Viruses. https://www.cdc.gov/flu/about/viruses-types.html. Accessed July 9, 2025.
  1. Cleveland Clinic. (2024). 1918 Influenza Pandemic (Spanish Flu). https://my.clevelandclinic.org/health/diseases/21777-spanish-flu. Accessed July 9, 2025.
  1. Martini, M., Gazzaniga, V., Bragazzi, N. L. and Barberis, I. (2019). The Spanish Influenza Pandemic: a lesson from history 100 years after 1918. J Prev Med Hyg. 60, E64–E67. doi: 10.15167/2421-4248/jpmh2019.60.1.1205
  1. Hershberger, S. (2020). The 1918 Flu Faded in Our Collective Memory: We Might ‘Forget’ the Coronavirus, Too. https://www.scientificamerican.com/article/the-1918-flu-faded-in-our-collective-memory-we-might-forget-the-coronavirus-too/. Accessed July 9, 2025
  1. Hajjar, S. A. and McIntosh, K. (2010). The first influenza pandemic of the 21st century. Ann Saudi Med. 30, 1–10. doi: 10.4103/0256-4947.59365
  1. Ratre, Y. K., Vishvakarma, N. K., Bhaskar, L. V. K. S. and Verma, H. K. (2022). Dynamic Propagation and Impact of Pandemic Influenza A (2009 H1N1) in Children: A Detailed Review. Curr Microbiol. 77, 3809–3820. doi: 10.1007/s00284-020-02213-x
  1. Leung, G. M. and Nicoll, A. (2010). Reflections on Pandemic (H1N1) 2009 and the International Response. PLoS Med. 7, e1000346. doi: 10.1371/journal.pmed.1000346
  1. Government of Canada. (2025). Influenza (Flu). https://www.canada.ca/en/health-canada/services/health-concerns/diseases-conditions/influenza-flu.html. Accessed July 9, 2025
  1. City of Toronto. (2025). Influenza (Flu) Fact Sheet. https://www.toronto.ca/community-people/health-wellness-care/diseases-medications-vaccines/influenza-flu-fact-sheet/. Accessed July 9, 2025
  1. Centers for Disease Control and Prevention. (2024). Healthy Habits to Prevent Flu. https://www.cdc.gov/flu/prevention/actions-prevent-flu.html. Accessed July 9, 2025.

By: Lisa Fang