Influenza (Flu): Seasonal Flu and Pandemic Flu

Life of the influenza virus

The influenza​ (flu) virus's main job in life is to make copies of itself. However, the virus is not very good at it. As the virus makes copies of itself, it makes mistakes called mutations.

Most of these mistakes hurt the virus and it dies or becomes unable to make copies of itself. Other mutations help the virus survive and make it easier for it to make copies of itself. A mutation may make it easier for the virus to spread from one person to another.

Small mutatio​ns

The flu virus makes copies of itself small but helpful mutations occur. After thousands of years, this process has resulted in many types and subtypes of flu. They live in a wide range of hosts, from pigs and birds to civet cats and humans. As hosts are infected and re-infected, they make antibodies to fight the virus which helps the host develop some immunity to the virus.

Because most of the virus's mutations are small, the host usually has some immunity to the slightly altered virus. This is because the antibodies that the host already has still have some ability to fight the slightly altered virus. The result is although people still get sick, their illness is milder and fewer people get very sick.

Larger mutatio​ns

Copying mistakes are not always small. Sometimes, large mutations occur. Large mutations may occur when two types of influenza that would normally be seen in different organisms infect the same host. For example, a small mutation may allow a pig to be infected with a flu virus normally only seen in birds. At the same time, the pig may also be infected with a flu virus that infects humans. When the viruses are making copies of themselves, genetic material from one virus can combine with the genetic material from the other. This can produce very large and sudden changes in the virus. It can create a new subtype of the virus.

If this mutation also helps the virus, hosts may have no natural immunity to it. This can cause more people to become infected. There may be more severe illness.

Evolution of a Pandemic Strain of Influenza
Get Adobe Flash player
This is a simplified explanation of the process that likely created the H1N1 virus strain (swine flu). Since the new virus is substantially different from the seasonal flu virus, humans have had little time to develop any immunity, making it potentially dangerous.

What is pandemic f​lu?

When a copying mistake is large and helps the virus, it makes the mutated virus different from the old virus. Because the virus is new very few people are immune to it. It is easier for this new virus to spread around the world and become a pandemic strain.

Pandemic flu can be mild or severe. It may infect many people or only a few. It may circulate for many months or die out. Predicting what will happen during a pandemic is very difficult. Pandemic strains of influenza have circulated at least three times in the last 100 years.

Most people have heard of the Spanish Flu of 1918. It killed tens of millions of people. But two other pandemic influenzas have circulated since then: Asian Flu in 1957 killed between one and four million people. Hong Kong Flu in 1968 killed approximately one million people.

The 2009 H1N1 pandemic flu infected millions of people. However, relatively few became severely ill. Over time, the virus will continue to mutate. It is difficult to predict what will happen with H1N1 in the future.

Key points

  • The flu virus makes copies of itself quickly and makes mistakes called mutations in its genetic material. These mutations may sometimes be helpful to the virus.
  • When two different types of flu virus exchange genetic material a new type of flu virus can be created.
  • If a new virus spreads fast and widely because people do not have immunity against it yet, it may become a pandemic flu.​

Upton Allen, MBBS, MSc, FRCPC, FAAP

Shawna Silver, MD, FRCPC, FAAP, PEng​​


Statement on Seasonal Influenza Vaccine for 2013-2014. (2013, November 14). Public Health Agency of Canada. Retrieved December 20, 2013.

Influenza (Seasonal) Fact sheet No211. (2009, April). World Health Organization. Retrieved December 20, 2013.

Seasonal Influenza (Flu) (2013, April 8). Health Canada. Retrieved December 20, 2013.

Key Facts about Influenza (Flu) & Flu Vaccine. (2013, September 26). Centers for Disease Control and Prevention. Retrieved December 20, 2013.

Protect yourself, your family and your community. (2013, October 31). Ministry of Health and Long-Term Care. Retrieved December 20, 2013.

Osterholm, M.T. (2005). Preparing for the Next Pandemic. New England Journal of Medicine, 352(18):1839-1842.