“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]


45 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

    1. Reading this article has provided me with some much need insight that I can share with patients, co-workers, family and friends. Thanks so much!

  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?

    2. Id like to see the proof of this. I’m not saying that you are wrong about that. Our government misleads us all the time. I bet you are a believer in aliens also. I know I am a believer. I’ve seen them.

  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.

      1. I think the flu/colds are spread more easily where there are lots of people in closed, confined spaces (major metropolitan areas) because it can more easily spread from one person to another! Plus, people’s immune systems differ depending maybe on genetics!

      2. because that is the capitol of satan. why would he want to spread disease where all his core employees rest?

  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..

    1. I get the exact same “tickle” feeling! Oddly, it’s only been in the past few years I’ve noticed this. If you start taking vitamin C every two hours at the slightest hint (sneeze, throat scratchiness, the tickle in the nose etc) – 500-1000 mg for an adult , and/or drink an antiviral herb tea (1 tbsp per 1 cup using thyme, basil or oregano) 3-4 times a day, this can (and most often does, for me) push off whatever it is. I had to fight these viruses/colds/flu like 7-8 times this season. I have no idea why the season should be so drastically different than it has my entire life, but most seasons I might fight a cold/virus or flu ONCE the entire season. Unfortunately this last time I felt a slight sore throat and managed to fight it completely off of me…late in the 2nd successful day of having zero symptoms, I suddenly picked it up again prob from the air (my husband is sick as well). This time I’m managing to push it off but I ran out of vitamin C so it’s been a bit more stubborn . I honestly can’t help but think that our bodies are meant to have much higher levels of C regularly throughout the day than we normally get. I’m not sure about the tiger balm. I’m assuming it has oils in it ; do viruses need to “breathe”?

  9. we are being had big style……………. flu virus is spread by flu shot uptake and mist………….. every single year i get a weird fluey thing now ,and it all started when they started pushing flushot etc……………. its as clear as day to me……… when i was a child hardly anyone i knew got flu or even too many colds,,,now, ALL winter” every one” almost has some full affliction or some “weird” long lasting thing alike a cold but not or flu but not ( thats the manmade cr#p that was in the shots that the uptakers shed for 14 days or so) people take this shot at different weeks too ,so it goes around and stays quite a while now……………. i wholey doubt warmer climate has anything to do with it… and even in winter when temperatures can drop quite low, i have NEVER heard a Turk or Greek say thy had the flu??? and believe me i do know many and have stayed there alot in winter …………………….. all this silly research ….. waste of time and money.

    1. Although I might agree with you that the flu vaccine is a waste of time and money I might not agree for the same reasons. The 1918 flu that devastated the world was before flu vaccines. Even tho flu vaccines were in effect during the one in the 60’s, still quite a few deaths laid at influenza’s door. I don’t think the vaccine is the sole cause of the virus wreaking havoc as the incidences of havoc predate the vaccinations. I’d place my bets with this author here:


    2. YES YES AND YES I Believe the flu is being worsened as they make flu shots every year, They are the reason for the crazy mutations each year!! I too never have heard so many flu problems when i was a kid, NEVER Its getting worse as they go, big phama wanting to make sure we all get flu shots and put money in their overloaded pockets!! Not to mention the sale of Tamiflu I hate them all

    3. Flu has killed a lot of people long before we had flu shots though. That said, it’s possibke imo that it may increase the numbers of people getting sick since it actually GIVES them a small dose of flu virus. The stupid thing about it (imo) is that very often the flu that is going around naturally isn’t even the same version the flu shot is intended to protect against (build immunities for). So then we have multiple types of flus gong around at the same time. I refuse to get flu shots. I have gotten the flu maybe twice in my life, the first as a child and the latest I caught from my husband who works in a large building with tons of exposure to these germs flying about. I honestly think the flu shot is probably doing more harm than good. This season (2018) we have heard of not just the elderly, very young and immune compromised being killed by these nasty viruses, but healthy young adults as well. One Dr said he had never seen a virus replicate so fast ! It killed a fairly young healthy woman within , I believe it was 24-48 hours. She was not an isolated case. Also some have been fighting one infection, only to contract another at the exact same time …so their immune system can’t keep up. It does seem to be getting much worse.

  10. Flu shots are BS. The human body can cure it’s self . We are supposed to build immunities to these viruses. Not inject ourselves with last year’s viruses. I have not had the flu in over 20 years and that was when I stopped getting the shot. I probably only had the flu shot 3 times in my life. My mom didn’t make me get it either. I got it a few times when a doctor told me that it was important. No more, no way. No one in my household gets it and none of us get the flu.

    1. Well, that settles it, then. Scheetz has spoken. Forget what all these silly “scientists” have to say. Anecdotal evidence by The Ms. Jennifer Scheetz? Etch it in stone!

      1. Your sacrcastic comment was uncalled for. She was only trying to help expose the truth about the flu shots! I too have never had the flu shot in a very long time, and it did not harm me one bit!

      2. I actually agree with her.
        The flu shot gives a person the virus in a small dose with the intent of building immunity. The problem is that they can guarantee that it will be the SAME flu going around that immunities protects against . This season there were multiple types of flu, including a version that transmits between humans and animals.

        I have NEVER gotten a flu shot and I have had the flu only twice in my entire life.

        You do realize that science and scientists are not perfect in their understanding right? Do you also realize that there are people who benefit monetarily from flu shots that “might” protect you from the flu. And actually, peoples experience tell us that many people get full blown cases of flu FROM taking the flu shot.

        And btw, regardless of your opinion, there is no call for being an ass just because you disagree with someone

    2. A lot of people have died. At least 75% have not had flu shots versus 25% who had a shot. This is amount reported in the DFW area.

      1. Please get your facts straight. The CDC counts any and all pneumonia deaths in those >65 in their MMWR stats on flu death. Scare tactics work to spread an agenda of increased profits for government/ Big Pharma. Please educate yourself on the influenza vaccine before you blindly jump on a bandwagon preventing anyone and everyone who disagrees with you the autonomy of declining to have what they view as poison shot into their bodies. I wonder how much women in the workforce (of which I am one) has contributed to the perceived “need” to prevent self limiting childhood illness that confer natural lifelong immunity. We have had numerous mumps outbreaks amid college students WHO HAVE BEEN IMMUNIZED in their youth….seems to be a false sense of security until kids reach an age where benign illness suffered as a child, contracted as an adult can either kill you or make you infertile…..hmmm, efficacious?? For whom?

  11. Unfortunately this doesn’t explain why tempered climate areas can have severe out breaks. Such as recently in California where they may have lower humidity but relatively warm temperatures. The South also has big out breaks in Florida where you have higher humidity and temperatures. Maybe as with other living species bacteria as we know it can adapt and survive in all climates even in space as we have now discovered. This may also be a reason why flu shots can be varied significantly in their effectiveness. This year Canada reported the flu shot effectiveness was at 17%. This is hardly a great statistic and explains people’s lack of confidence in the shot. I think the better solution is to prevent spread by immediately isolating people who have signs of the flu. Even someone with the flu that shows up for work or school can spread the bacteria quickly. Too many trying to tough it out and stay working or parents pushing sick kids out the door is contributing to the spread.

    1. I agree! Keep people segregated until they’re not infectious. If it’s killing people and you’ve got it then stay away from others while you’re sick. People with AIDS aren’t allowed to give it to others without consequences, so why would ppl who knowingly have the flu be allowed to go into public without consequences?

    2. It doesn’t help that in spite of employers saying they want employees to stay home while sick so as not to spread the virus, flu can be contagious for…what, 7 days from the time it’s contracted (and one day before symptoms show up ) …and most employers might be ok with an employee taking one or two days off but God help you if you want to take a full 7 days in order to protect others & prevent it spreading.

      The truth is that people can’t afford to stay home without pay for a week (some viruses can take TWO full weeks before the person is no legal beer contagious) AND employers are not really willing to let them even if they could. It’s unfortunate, but the way our society is, these viruses are just going to keep spreading and killing people every winter. It’s shocking really when we realize that 35-36,000 people DIE every year from the Flu – especially when we realize it could be prevented. I hate to say it, but it would probably be best if the vast majority of people worked from home in winter – I know, not really feasible. Too bad we couldn’t do that and stock up for enough food supples to last the winter.

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