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The word flu derives from the Italian phrase "influenza de freddo" meaning "influence of the cold".

Indeed it is that time of the year when my colleagues seem to have the flu/cold more often than not. I remember during university, winter lectures would be riddled with people coughing and sneezing - the so called "freshers flu"

Questions

  • Is there statistical evidence for this claim?
  • If so, what are the causes/factors involved?
  • Another post asks why our noses get runnier during cold whether, is this related?
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    $\begingroup$ As a sidenote: looking at the kind of questions you ask, you might be interested in this book $\endgroup$
    – Remi.b
    Commented Dec 3, 2013 at 9:49
  • $\begingroup$ Cheers, I think so far you, @GriffinEvo and biogirl have suggested too many books for my wallet to handle. Will keep this in mind until i progress through them :) $\endgroup$
    – hello_there_andy
    Commented Dec 3, 2013 at 17:10
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    $\begingroup$ @Remi.b Oh yes, I read about that book in the book "the blind watchmaker" and Dawkins says it's nice too. :) $\endgroup$
    – biogirl
    Commented Dec 6, 2013 at 9:25

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The combination of these two reports from the CDC give information about the comparative prevalence of flu infection in the winter (September '12- May '13) and summer (May '13 - September '13). I'm going to assume that 2012-2013 was a fairly representative year as far as the level of detail of "do we get sick more in the winter" goes. Particularly striking are the graphs linked to in both documents of the number of flu-infected respiratory specimens per week:

Winter (infected specimens out of 311,000 tested):

enter image description here

Summer (infected specimens out of 52,000 tested):

Summer influenza cases

Together these two figures cover data Sept '12 - Sept '13. Note the difference in scale on the vertical axis. Forgive me if I don't run any statistical tests, but I think you can spot the trend.

The sum of all the Winter cases represents 23% of the 311,333 specimens tested from Sept '12 to May '13, and the sum of all the bars in the Summer figure represents 3.9% of the 52,150 respiratory specimens tested from May '13-Sept '13.

Still left to answer: Why is this the case? In my preliminary research it seems like we still don't fully know the answer to this one; I'll leave someone else to speculate.

Edit: Some tentative answers to why is there a seasonal flu outbreak:

This review (1) does a nice job of laying out some of the current ideas about the cause of the seasonal flu. One interesting note is that seasonal flu is a phenomenon of temperate climates: in tropical regions flu outbreaks occur on a much less regular periodic schedule. Some possible causes mentioned by the authors (paraphrased from the article):

  • It takes time for virus to mutate enough to be infectious again to people who were already infected with it.
  • The efficacy of the immune system may vary seasonally, say, due to changes in daylight hours/vitamin D and melatonin levels (which have been shown to affect immune system function in hamsters)
  • Flu is transmitted better in more crowded conditions + in winter people are more crowded together. This is a popular explanation, but the authors claim that it has not been satisfactorily tested. It has been argued convincingly that the 1918 flu epidemic was exacerbated by the crowded condition of army barracks in WWI (see citation in paper).
  • Indoor heating encourages air recirculation, which allows virus to persist in the air around you longer.
  • Viruses survive/persist better in cold, dry conditions. This may allow them to spread in the atmosphere longer during the winter, and be more easily transmitted from person to person via "bioaerosols" (sneezes).

Ultimately, as I said before, we don't really know yet. Reading that list you can imagine several ways in which a few of those factors are not at all causal and are rather just strongly correlated with the "true" cause of flu seasonality.

(1): Lofgren et al. Influenza Seasonality: Underlying Causes and Modeling Theories. (2007). J. Virol. 81(11): 5429-5436. doi:10.1128/JVI.01680-06

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    $\begingroup$ As this answer cites my paper, it is clearly correct ;). I would note that it's true for influenza, and a few other winter-circulating diseases (Norovirus comes to mind). Other infections are more prevalent in the summer (many bacterial GI infections for example) and some don't care at all. $\endgroup$
    – Fomite
    Commented Dec 3, 2013 at 22:14
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    $\begingroup$ "Viruses survive/persist better in cold" - Could you provide proof links on that? $\endgroup$
    – Stanislav Bashkyrtsev
    Commented Jan 25, 2014 at 19:22
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    $\begingroup$ @ctapobep, I am mostly just paraphrasing the open-access article cited here. In that article they make the claim several times that viruses are more stable in cold, dry environments. They cite this paper: Hemmes et al. (1960). Virus Survival as a Seasonal Factor in Influenza and Poliomyelitis. Nature 188: 430-431, which appears to investigate the stability of certain viruses in cold, dry environments. $\endgroup$
    – A. Kennard
    Commented Jan 27, 2014 at 13:27
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    $\begingroup$ @ctapobep, furthermore, This result isn't surprising: viruses are inert particles made of proteins in a relatively ordered/low entropy configuration. If you increase the temperature, you make it more thermodynamically favorable for the protein making up the viral particle to loosen from their ordered configuration and move freely, which effectively destroys the particle. $\endgroup$
    – A. Kennard
    Commented Jan 27, 2014 at 13:29
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    $\begingroup$ Has anyone investigated the possibility of migrant birds carrying new strains? $\endgroup$
    – Superbest
    Commented Apr 21, 2014 at 21:02

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