Talking Evolution – The Challenge of Influenza – Part 2

In my previous post, I detailed the nature of the influenza virus and its impact on personal health. In this post, we’ll look at how and why this virus is such a challenge for humanity.

Despite prodigious advancements in medical care as well as vaccine technology, we still face yearly health hazards from seasonal influenza, as well as the generational threat from occasional influenza pandemics. By looking at these global pandemics, we can appreciate the evolutionary forces that allow new forms of influenza to appear and the cultural factors that enhance their ability to spread globally.

Image courtesy the Centers for Disease Control (CDC)

Let’s take a quick look at the major influenza pandemics of the past 100 years.

  • 1918: The “Spanish Flu” killed as many as 50 million people worldwide. The spread was associated with troop movements related to World War I. The responsible virus was of the H1N1 subtype.
  • 1957: The “Asian Flu” killed between one and two million people as it swept out of China and through Southeast Asia and Japan before covering the globe in less than six months. The virus in this pandemic was an H2N2 subtype.
  • 1968: The “Hong Kong Flu” erupted from Hong Kong, spread to Taiwan, and then followed American troops home from Vietnam. Between 30% and 40% of people in the United States became infected with this strain, which was a unique H2N2 subtype. Between half a million and a million people died in this pandemic.
  • 2009: The H1N1 Swine Flu Pandemic emerged from Mexico and spread around the world with unusually variable severity—some areas were hit hard while others were barely affected. One of the most vulnerable populations was young adults, unusual given children and the elderly are frequently the hardest hit. Deaths from this strain were estimated at close to 200,000.

Strains of influenza that circulate in human populations change slightly from year to year and require updated vaccine production. However, these pandemic strains of influenza have a fundamentally different method of change at work.

Influenza: Get the (Antigenic) Drift by National Institute of Allergy and Infectious Diseases

The small-scale annual changes in the virus is referred to as “antigenic drift”—the process by which a series of small mutations alter the H or N molecules on the surface of the virus. I relate this to my students as a person changing the color (but not the style) of their shirt.

With the pandemic versions of influenza,changes make the virus barely recognizable to the immune system. It’s as if the person is changing the style of their shirt from a Hawaiian shirt to a down parka. The method of this change is referred to as “antigenic shift.”

Antigenic shift occurs when the genetic information of the influenza virus undergoes multiple changes due to either mutation or by the shuffling of genes with other circulating versions of the influenza virus. Antigenic shift seems to be tied to important geographic and cultural factors.

Students can make connections among these geographic and cultural factors contributing to the genetic recombining of virus types that leads to antigenic shift. One useful strategy is to have them scan a series of news articles about historic flu pandemics and identify recurring terms. By doing this, students quickly pick up that most pandemic strains have some sort of Chinese/Asian origin and the terms “swine”, “bird”, and “avian” appear more often then they would expect. Once students see this, they are on track to making the big connection between biology and culture.

This graphic is a great representation of how antigenic shift occurs.
Image by Giulio Frigieri & Paul Scruto, courtesy the National Institute of Allergy and Infectious Diseases

Once these connections are pointed out, I can ask students about how the human geography of China and Southeast Asia might relate to birds and pigs.

This line inquiry leads to the understanding that in agricultural areas of China and neighboring regions, many pigs and ducks live in close proximity to their human farmers. As mentioned in my previous post, influenza A can infect all of these species. With pigs, birds, and human in close proximity, the strains of influenza A that may have been isolated in one species now have the chance to infect others.

Additionally, with multiple strains in one animal, the genes from each strain have the chance to get swapped or recombined into a new version of influenza. Such a change can expose humans to combinations of the H and N molecules that our immune systems are unfamiliar with, and thus place healthy people in danger if the new version is capable of spreading easily.

Flu Shift and Drift by Khan Academy

When thinking about this method of producing new influenza strains, it helps to consider that two of the three species involved (humans and birds) are quite mobile and can easily spread the new strains from a rural location to more urban population centers. There, they can infect people who have the capacity to travel globally. As such, we have to realize that the time between the appearance of a new strain of the flu and every major population center is potentially just days apart.

Thankfully, epidemiologists (scientists who study global health issues and disease patterns) now recognize Southeast Asia as a hotspot for new influenza strains and monitor both antigenic drift and shift in birds, pigs, and people throughout the region. This allows scientists to sound the alarm when potentially dangerous strains appear. Such knowledge allows the Centers for Disease Control (CDC) to plan what strains of influenza to include in the next year’s vaccine model.

Currently, flu vaccines are based on either three (trivalent) or four (quadrivalent) strains of influenza. Such a mixture allows protection from both the most common strains and the most deadly newcomers.

Image courtesy the National Institute of Allergy and Infectious Diseases

While we focus on Southeast Asia as an influenza “nursery”, in 2009 we learned that pandemic influenza can emerge from other places. When the 2009 H1N1 pandemic hit the United States, scientists traced its origins not to Asia, but to central Mexico.

Central Mexico does not have the “triple threat” of resident humans, pigs, and waterfowl that Southeast Asia does, but when the genetic structure of the 2009 H1N1 strain was analyzed, it seemed to have combined genes from Mexican swine flu as well as segments from Eurasian swine flu varieties.

How did a Mexican flu strain pick up genes from a Eurasian flu strain? Well, it seems that pigs can fly!

Due to livestock trade between Europe and Mexico, pigs carrying Eurasian strains of influenza were brought to central Mexico. There, the pigs shared infections which led to the appearance of the novel 2009 H1N1 pandemic.

Thankfully, close monitoring and vaccine production limited the mortality of this new strain to death numbers comparable to annual seasonal influenza. The 2009 strain has since become the most common circulating strain in existence here in 2017.

The flu is easily overlooked, but this disease provides a great teaching opportunity to blend geography, evolution, epidemiology, cultural norms, genetics, and virology into a lesson that connects student health with deeper human history.

Remember: Great education is not about fact-collecting, but about teaching our youth to connect facts in ways that generate meaning beyond mere factual tidbits.

Have YOU had your flu shot yet? Influenza is evolving, so make sure you keep pace to remain healthy and flu-free!

Useful flu resources: 

Centers for Disease Control Influenza pages –

NIAID : How Influenza Pandemics Occur –

WHO: Influenza, an Unpredictable Threat –

Khan Academy Flu Playlist –

The Mexican Origin of 2009 H1N1 Influenza –


In Talking Evolution, teacher and science communicator John Mead brings evolution and biogeography “down to earth” with practical ideas for classrooms and learning networks.

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