As World War I ended, a new war began. The flu pandemic of 1918 spread across the world. It was first identified in the U.S. in soldiers after returning from the war. Because there were no vaccines, or antibiotics to treat secondary bacterial infections, the pandemic spread to infect 500 million people and kill 50 million — 3% of the global population — making it the deadliest pandemic in history, second to the Black Death.

In just twenty years, immunologist Jonas Salk and his New York University mentor Thomas Francis developed the first flu vaccine. Their discovery, which was first tested on U.S. soldiers in World War II, laid the groundwork for Salk’s later work on a polio vaccine.

By the mid 1940s, the vaccine became publicly available and researchers began to notice a troubling trend: For reasons that are still not entirely understood, the genes of the flu virus tend to “drift” year to year, creating slight mutations that can render last year’s vaccine ineffective. That’s why the flu season is a never-ending story of immunological mystery, in which scientists at the World Health Organization (WHO), U.S. Centers for Disease Control (CDC), and other agencies make informed guesses about what change may occur and preemptively design a new vaccine to protect against the new viral strains.

Considering the challenge of the task, they’ve been remarkably successful: The seasonal death rate from flu in the U.S. in the 1940s was around 10 per 100,000 people; by 2017, it was around two. However, the disease still infects up to five million people globally each year.

It is clear that influenza is highly contagious and can spread quickly – typically by the coughing and/or sneezing of an infected person. Yet people cough and sneeze year round, so why does it appear that influenza also travels with the seasons? 

A common misconception is that the flu is caused by cold temperatures. Colder weather is not a cause, but a contributing factor. During the winter months, people spend more time indoors with the windows sealed, so they are more likely to breathe in the same air as someone who has the flu and may contract the virus. Remember, it is transmitted by airborne particles (coughing, sneezing, sharing food, etc.). The shorter days of winter also result in lack of sunlight. This contributes to a decreased ability of the body to make vitamin D and melatonin. These low nutrient levels compromise our immune systems, which decreases our ability to fight the virus.

There is another contributing factor to why cases of influenza rise during the winter season – lifestyle. Around the end of October, as the weather officially changes, so does the average lifestyle. From Halloween to New Years, our lifestyles increase in stress and consumption of sugar. This, combined with decreased sun exposure, is a major factor in why we are more at risk of influenza during the winter months. 

This is good news! Because we are in control of our lifestyles, we can actually boost our immunity during these times and say good-bye to flu “season!”  


Belser, JA ; Tumpey, TM. The 1918 flu, 100 years later: Science. 2018 Jan. Vol. 359, Issue 6373, pp. 255external icon

 Centers for Disease Control. The Flu Season. <http://www.cdc.gov/flu/about/season/flu-season.htm> [2 November, 2014]

Lowen, A.C., S. Mubareka, J. Steel, and P. Palese. 2007. Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature. PLOS Pathogens. 3(10):e151

Roos, R. 2013. Study: Flu likes weather cold and dry or humid and rainy. University of Minnesota Center for Infectious Disease Research and Policy. <http://www.cidrap.umn.edu/news-perspective/2013/03/study-flu-likes-weather-cold-and-dry-or-humid-and-rainy> [14 November, 2014]

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