It’s that time of year again. It’s “Flu Season”—and those words mean different things to different people.
For some of us, “flu season” is synonymous with the idea that we might come down with a “cold” and feel crummy for a few days. For a smaller group, it means that we are at risk of a life-threatening illness.
As a teacher of middle school students, I know that young people are generally unaware of what influenza is, and the actual threat it poses to human health.
In this two-part post, we will look at what “the flu” really is and how it affects our health.
In classroom experience over the past 28 years, I find that the vast majority of middle-school students view influenza as an annoyance illness that is just slightly worse than the common cold. Given their generally uninformed state on this topic, I now take it as an opportunity to teach an interdisciplinary lesson that connects virology, immunology, and epidemiology with evolution, agriculture, and patterns of human migration and travel.
I relish any chance I get to demonstrate that science is not a collection of facts to be memorized and regurgitated. Rather, science is a framework of facts that when connected to each other provide a meaningful method for understanding the natural world. Influenza education is just such an opportunity.
science is a framework of facts that when connected to each other provide a meaningful method for understanding the natural world
To get all of us on the same wavelength, it helps to see statistics about the impact of influenza in the United States over a period of time. When you look at the Center for Disease Control’s (CDC) annual averages since 2010, you can see the number of affected people is both variable and substantial. The average yearly number of influenza cases has ranged between 9.2 million and 35.6 million patients. This yielded an average of 140,000 to 710,000 hospitalizations for influenza each year and an annual death toll of 12,000 to 56,000 people.
While these numbers make influenza a top-10 cause of death each year, these numbers pale in comparison to the numbers of deaths caused by influenza when new forms of it create worldwide pandemics—the most infamous of these being the “Spanish Flu” outbreak of 1918 when an estimated 20 million to 50 million people (approximately 5% of the world’s population!), including 675,000 Americans, died in the course of a single flu season.
In order to better understand the cause of such suffering and death, let’s take a look at what influenza really is and how it impacts people under normal circumstances.
First off, the influenza viruses consist of a grouping of four virus types. These are classified by the letters A, B, C, and D. Humans are infected by influenza A, B, and C. Influenza D is a problem for cattle, but does not have the capacity to infect people. Influenza C infects people but produces a mild respiratory illness and does not spread widely.
When you and I talk of the seasonal flu, we are thinking of the disease caused by the A and B types. Of these two, type A is the most variable version and has the capability of also infecting pigs (swine), birds, horses, dogs, and cats. Type B only occurs in humans and seals. The fact that influenza A infects so many species will be a hugely important aspect of why it causes so many issues for humans!
As we head into the 2017-18 influenza season, the most prevalent types of influenza in the United States are currently (in early December 2017) two versions of influenza A (H1N1 and H3N2) and two strains of influenza B (B/Victoria and B/Yamagata)
The way humans (and other animals) get infected by influenza viruses involves the interaction between the structure of the virus’ outer protein coat and the surface of the cell the virus infects.
This interaction involves two viral molecules. The first we call the H number (for the type of hemagglutinin glycoprotein) and the second is the N number (for the type of neuraminidase glycoprotein). There are 18 different H molecules and 11 different N molecules. As such, a virus is referred to by its H and N molecules. Many of you are familiar with the H1N1 type which means those virus particles have H molecule #1 and N molecule #1.
The hemagglutinin molecule on an influenza virus particle serves to help connect the virus with a part of the target cell membrane called sialic acid. When the correct hemagglutinin combines with sialic acid, the virus is absorbed by the target cell which allows the virus to begin to replicate by the millions in that target cell.
Once the virus particles are copied within the cell, the molecules of neuraminidase serve to make it easier for the new flu virus particles to escape from the host cell in massive numbers. These new virus particles can now infect neighboring cells throughout the respiratory system.
This video from NPR is a great overview of the process of infection.
Once the flu (or any other) virus appears in your body for the first time, your immune system will be slow to recognize the foreign proteins on the surface of the virus. This gives influenza a several-day head start to produce millions of new viruses before your own immune system can ramp up production of antibody proteins against the specific type of flu. As such you get sick with the symptoms of the flu (fever, coughing, sore throat, runny/stuffy nose, muscle/body aches, headaches, and fatigue).
Once your antibody production catches up with viral production, your symptoms diminish, and you begin to recover. The average time people suffer from flu symptoms is between three and six days. Regardless of how you feel, you can be contagious the day before symptoms appear and remain so for up to a week.
Hank Green of the YouTube channel Crash Course has a great visual explanation of the antibody-producing process that’s worth your time.
While a case of the flu makes anyone miserable, there are two groups at significantly higher risk for complications and even death from influenza.
These groups are infants and young children who have yet to develop strong immunity, and the elderly who may have weaker immune systems or other underlying health concerns.
To protect both these groups and the general public, the Centers for Disease Control (CDC) recommends that EVERYONE older than six months of age get vaccinated against the flu every year.
The fact that the CDC recommends us to get the flu vaccine every year is unusual and reflects the fact that influenza viruses rapidly evolve—taking on differing H and N molecule configurations. As such, last year’s vaccine may or may not be useful against a newer strain of influenza.
In the next post, we’ll look at the unique biological, geographical, and cultural factors that allow influenza A to mutate so successfully year after year.
Useful Influenza Resources
All things influenza-related from the Centers for Disease Control – https://www.cdc.gov/flu/index.htm
Influenza information from the World Health Organization (WHO) – http://www.who.int/mediacentre/factsheets/fs211/en/
Website of the upcoming (January 2018) documentary recognizing the 100th anniversary of the Influenza outbreak of 1918 – http://www.pbs.org/wgbh/americanexperience/films/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.