“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]


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

  1. Maybe cold weather caused immunity to fall, survival of influenza is just red herring.

    That’s why in tropics, as you said, rainy and humid weather caused influenza cases to increase- rain means colder weather in tropics too.

    Cold is energy sapping, so a relative drop in temperature is likely to cause immunity to fall and thus susceptibility to influenza will increase during the colder months of the year.

  2. Covid!It may be designed purposely to decrease the surplus population…Special attention to non working seniors and sickly persons.);

  3. The flu Is not contagious. This is well documented in the roseanu experiment of 1919 in JAMA magazine . Also in the book the invisible rainbow proves without a doubt that the flu is not contagious. Flu season is just that, a season. Contagion does not have perfect seasons. Since the flu has been proven not to be contagious at all, it puts a real question on the plandemic hoax. Covid is nothing more then the reg flu rebranded. It’s all a hoax to instate the new world order. It’s the new never ending story. Masks don’t do a darn thing to prevent anything. The world has been sold a lie and it’s not the biggest one. In a nutshell everything we been taught is backwards and most won’t understand this but it’s true. Bill gates said that vaccines will lower the population in 2013 during his Ted talk. The powers that be want to depopulate the world. They certainly don’t want to save anyone with their poison vaccines.

  4. Let’s say I don’t have the flu and I’m going to live on a desert island. How do I get the flu if there is no one to infect me?

    1. Become the flu is a natural cycle of detoxing. It’s not contagious at all. Have you ever heard of the roseanu experiment? This proves without a doubt the flu is not contagious. Also the book the invisible rainbow also proves this. Contagion does not have perfect seasons. Do people think anymore?

  5. I strongly suggest that the scientific community investigate the impact of H202 in the atmosphere on the influenza seasonal variations. It better explains why the seasonal variations in flu occur in warmer climates also.


    Very interesting article. The only thing I partially disagree with is the last sentence, which states that “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.” Every Cochrane Review I’ve seen about influenza vaccines states that they are either ineffective, or so marginally effective that they have very little impact on rates of flu infections. For instance, I was just looking at a Cochrane Review published in 2014, titled “Vaccines for preventing influenza in healthy adults.” According to the review: We included 90 reports containing 116 data sets; among these 69 were clinical trials of over 70,000 people, 27 were comparative cohort studies (about eight million people) and 20 were case-control studies (nearly 25,000 people). We retrieved 23 reports of the effectiveness and safety of vaccine administration in pregnant women (about 1.6 million mother-child couples)…. 2.4% [of unvaccinated] and 1.1% [of vaccinated people in the studies] developed laboratory-confirmed influenza. Although I am a sample size of only one, and my experience neither proves nor disproves anything, I have not had a flu shot in decades (even when I was married and living with my wife and kids), and I have never contracted the flu since I stopped getting flu shots. It seems that keeping warm and using a humidifier at home is the best way to avoid contracting the flu only at home. But the likelihood of contracting the flu is probably highest in indoor, public locations where the density of people is highest and/or the likelihood of close contact is highest, and the humidity level is lowest. Once someone contracts the flu in such a public location, a humidifier at home would probably not be helpful. Since I live alone, it would make no sense for me to invest in a humidifier. But the larger one’s family is, and the greater the amount of time each family member spends exposed to other people indoors in low-humiidity environments during the winter, the more sense it makes to invest in enough humidifiers at home to ensure that all areas of the house/apartment where family members come into contact with each other have high humidity levels (with higher heating bills). Of course, a humidifier at home would not benefit someone who contracted the flu outside the home (except indirectly, by reducing the likelihood that said person would infect family members at home).

  7. Isn’t it curious that guinea pigs are discovered to share the vulnerability to influenza? Aren’t they one of the few (only?) mammals not to produce their own vitamin C, just like humans?

  8. John Hunter (and others) are correct in their view that the mere description of virus infectivity as a function of temperature and humidity – though alas the only experimental data besides the observation of seasonality – need to be expanded by mechanisms that explain these infectivity data. As virus particles leave the infected bodythrough nose or mouth they are contained in a waterous solution with or without mucus. Once in the air, aerosols can be expected to evaporate the water leaving either completed dry viruses or viruses with shallow water coating (while large droplets tend to sink rapidly). In a superspreader event of a choir singing in a closed room, the former (aerosol) situation is to be expected. Here, it would be important to know if “wet” or “dry” viruses are more infective – both in the flu and the Corona virus situations. Have any experiments been performed or are there theoretical considerations?

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