“Did you get your flu shot?” If your friends are anything like mine, you heard this question at least a dozen times before Thanksgiving. You probably got your fair share of disdainful looks too, if you answered “No.” But why are we worried about getting the flu shot now and not in May? Why is there a flu season at all? After all, what does a virus living in a host who provides a dependable, cozy incubation chamber of 98°F, care whether it is freezing and snowy outside or warm and sunny? This question has bothered people for a long time, but only recently have we begun to understand the answer.

What is the Flu?

In order to discuss why we have a flu season, we must first understand what the flu is. The flu, also called influenza, is a viral respiratory illness. A virus is a microscopic infectious agent that invades the cells of your body and makes you sick. The flu is often confused with another virus, the common cold, because of the similarity in symptoms, which can include a cough, sore throat, and stuffy nose. However, flu symptoms also include fever, cold sweats, aches throughout the body, headache, exhaustion, and even some gastro-intestinal symptoms, such as vomiting and diarrhea (1).

The flu is highly contagious. Adults are able to spread the virus one day prior to the appearance of symptoms and up to seven days after symptoms begin. Influenza is typically spread via the coughs and sneezes of an infected person (1). Around 200,000 people in the United States are hospitalized each year because of the flu, and of these people, about 36,000 die.  The flu is most serious for the elderly, the very young, or people who have a weakened immune system (1).

The Flu Season

The flu season in the U.S. can begin as early as October, but usually does not get into full swing until December. The season generally reaches its peak in February and ends in March (2). In the southern hemisphere, however, where winter comes during our summer months, the flu season falls between June and September. In other words, wherever there is winter, there is flu (3). In fact, even its name, “influenza” may be a reference to its original Italian name, influenza di freddo, meaning “influence of the cold” (4).

A common misconception is that the flu is caused by cold temperatures. However, the influenza virus is necessary to have the flu, so cold temperatures can only be a contributing factor. In fact, some people have argued that it is not cold temperatures that make the flu more common in the winter. Rather, they attest that the lack of sunlight or the different lifestyles people lead in winter months are the primary contributing factors. Here are the most popular theories about why the flu strikes in winter:

1) During the winter, people spend more time indoors with the windows sealed, so they are more likely to breathe the same air as someone who has the flu and thus contract the virus (3).

2) Days are shorter during the winter, and lack of sunlight leads to  low levels of vitamin D and melatonin, both of which require sunlight for their generation. This compromises our immune systems, which in turn decreases ability to fight the virus (3).

3) The influenza virus may survive better in colder, drier climates, and therefore be able to infect more people (3).

 The Flu Likes Cold, Dry Weather

For many years, it was impossible to test these hypotheses, since most lab animals do not catch the flu like humans do, and using humans as test subjects for this sort of thing is generally frowned upon. Around 2007, however, a researcher named Dr. Peter Palese found a peculiar comment in an old paper published after the 1918 flu pandemic: the author of the 1919 paper stated that upon the arrival of the flu virus to Camp Cody in New Mexico, the guinea pigs in the lab began to get sick and die (4). Palese tried infecting a few guinea pigs with influenza, and sure enough, the guinea pigs got sick. Importantly, not only did the guinea pigs exhibit flu symptoms when they were inoculated by Palese, but the virus was transmitted from one guinea pig to another (4).

Now that Palese had a model organism, he was able to begin experiments to get to the bottom of the flu season. He decided to first test whether or not the flu is transmitted better in a cold, dry climate than a warm, humid one. To test this, Palese infected batches of guinea pigs and placed them in cages adjacent to uninfected guinea pigs to allow the virus to spread from one cage to the other. The pairs of guinea pig cages were kept at varying temperatures (41°F, 68°F, and 86°F) and humidity (20%-80%). Palese found that the virus was transmitted better at low temperatures and low humidity than at high temperatures and high humidity (see Figure 1).

Figure 1 ~ Experimental Setup. Guinea pigs were housed in adjacent cages. Guinea pigs in cage 1 were infected by Palese with influenza. Palese observed how many guinea pigs in cage 2 became infected from the guinea pigs in cage 1 at different temperatures and levels of humidity. B, C) Transmission rates were 100% at low humidity, regardless of temperature. At high humidity, transmission occurred only at the lower temperature. 

However, Palese’s initial experiment did not explain why the virus was transmitted best at cooler temperatures and low humidity. Palese tested the immune systems of the animals to find out if the immune system functions poorly at low temperatures and low humidity, but he found no difference in innate immunity among the guinea pigs (5). A paper from the 1960s may provide an alternate explanation. The study tested the survival time of different viruses (i.e. the amount of time the virus remains viable and capable of causing disease) at contrasting temperatures and levels of humidity. The results from the study suggest that influenza actually survives longer at low humidity and low temperatures. At 43°F with very low humidity, most of the virus was able to survive more than 23 hours, whereas at high humidity and a temperature of 90°F, survival was diminished at even one hour into incubation (3).

The data from these studies are supported by a third study that reports higher numbers of flu infections the month after a very dry period (6). In case you’re wondering, this is only the case in places that experience winter. In warmer climates, oddly enough, flu infection rates are correlated most closely with high humidity and lots of rain (6). Unfortunately, not much research has been done to explain these contradictory results, so it’s unclear why the flu behaves so differently in disparate environments. This emphasizes the need for continued influenza research. Therefore, we can conclude that, at least in regions that have a winter season, the influenza virus survives longer in cold, dry air, so it has a greater chance of infecting another person.

Although other factors probably contribute as well, the main reason we have a flu season may simply be that the influenza virus is happier in cold, dry weather and thus better able to invade our bodies. So, as the temperature and humidity keep dropping, your best bet for warding off this nasty bug is to get your flu shot ASAP, stay warm, and invest in a humidifier.

Hannah Foster is a PhD candidate in the Molecules, Cells, and Organisms program at Harvard University. 

For more information about the flu, check out this video:


1) Medical News Today. What is flu? What is influenza? What are the symptoms of flu? < >[2 November, 2014]

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

3) Elert, E. 2013. FYI: Why is There a Winter Flu Season? Popular Science.  <http://www.popsci.com/science/article/2013-01/fyi-why-winter-flu-season> [2 November, 2014]

4) Kolata, G. 2007. Study Shows Why the Flu Likes Winter. New York Times. <http://www.nytimes.com/2007/12/05/health/research/05flu.html?_r=1&> [2 November, 2014]

5) 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.

6) 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]


15 thoughts on “The Reason for the Season: why flu strikes in winter

  1. thank you for interesting article. Please can you be more specific, in the graphics (and text) you do not specify if it is relative or absolute humidity. However, since expressed in % one may assume it is relative humidity. On the basis that the absolute humidity is important to organism, then lower temperature means lower absolute humidity at given relative humidity. Otherwise put, lower temperatures are MUCH drier than warm at same relative humidity. The temperature being colder accentuates the humidity effect. Thank you for contact by email with any useful publications.
    regards, n

  2. I think that in Russia (looking and watching what is gaoing on, mosto people are ill, doctors can’t say that the desease is what) the virus is spread by our Government to male the sales of drugstores. I think that it is the imune system that is working. I don’t get used to the tablets and was cured by berries and the same in the childhood. I was surrounded by virused people including my mom but there’s no virus in me.

  3. The common cold is a man-made virus which is released every year into the populous around Autumn time in various heavily populated major towns around the world. The untold truth is that this is a way of governments to control the ever aging and growing population of the world. I have personal evidence of dealing with the secret organisation that is responsible for population control. The proof is out their in-front of everyone’s eyes in black and white, I mean come on how is it that everyone seems to get the flu or colds in winter???? think about it! there’s more chance you’ll kick the bucket due to colder weather, therefore yielding a higher mortality success rate. Its a cruel world but the truth needs to be told!

    1. How did people get colds before we had the technology to manipulate DNA? How did people get colds in the 1800s? In the 1700s?

  4. Please do explain whether in warmer cities in the USA – for example, Los Angeles, San Diego – people suffer fewer colds than, say, in New York City, Boston, etc. during the winter months. And, are there any places in the world where people almost never catch colds and/or the flu? What about parts of the world where the flu is much more common than the rest of the world? Thank you very much.

  5. Are colds and the flu as common in San Francisco, CA (relatively milder climate most of the year) as in New York City (relatively much colder in the winter months, but somewhat warm/hot and humid in summer compared to San Francisco)? Thank you very much.

  6. i would like to address your hypothesis that the reduced hours of sunlight during the winter negatively affect people and could contribute to flu transmission.

    While there *are* fewer hours of sunlight during the winter, other factors also affect the manufacture of vitamin D and melatonin. First of all, during winter, the sunlight strikes higher latitudes at a much greater angle of incidence. Imagine that you are standing on the equator at noon on the spring or fall equinox. Imagine that the sunlight falling on you is a long tube of one foot radius connecting you to the surface of the Sun. The sunlight comes straight down, and hits the ground and shines on an area of pi square feet. The energy is very concentrated.

    But imagine instead that you are standing in Chicago, at 43ºN latitude on the winter solstice, when the Sun is directly overhead at a point 22.3ºS latitude. The sunlight falls on you at 64º off from straight overhead. The formula for the area of an ellipse formed by cutting a cylinder of radius r at angle α is A = pi * r^2/ sin(α). For a cylinder of 1 foot and an angle of 64º, this gives us an area of 1.11 square feet, for an 11% increase in area or a 12% decrease in energy because it is spread over that increased area.

    All of this assumes that you are spending just as much time outside, which of course is generally not true. In the summer your typical cubicle worker is eager to spend the weekend outside if the weather is nice. In the winter, even if it is sunny, the average person won’t spend all day at the beach when it’s 10ºF. Plus, in the winter, even if you are outside, you are bundled up. At 10ºF, a person outside will wear clothing over all of their body except maybe their face. This is entirely different from the clothing worn when it’s 80ºf outside, in which 50-90% of the body is uncovered.

    In short, it is not just the hours of sunlight available that are reduced in the winter.

  7. In our state (Queensland, Australia), the tropical part (Cairns, Townsville etc.) gets ‘flu at the time as the sub-tropics (Brisbane etc.) and plenty. There is certainly no shortage of ultraviolet light in Cairns in the Winter, and the temperature rarely falls below 20C, even at night (26-30 during the day). Seems to me there must be some other explanation than climate factors (temperature or humidity). It astounds me that we still haven’t figured this out!

  8. I’ve never had flu, touch wood.. But every time I get a cold, I immediately know as I feel a certain tickle sensation in one of my nostrils that is different to anything else. I know instantly when a cold has ‘landed.’ However, If I’m lucky enough to have tiger balm on me, (which I use for headaches) and I place the tiger balm on the spot inside the nostril, the cold never lasts more than one day. If I don’t do this, (no tiger balm available at the right moment) I’m in for the long haul.. Can you tell me why this might work. I’m not scientific in any way, lol..

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