# Why is winter the season for colds, flu, etc.? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [March 22, 2013, 3:10pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641 "2013-03-22T15:10:32Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![eldowan](https://avatars.discourse-cdn.com/v4/letter/e/848f3c/32.png) [@eldowan](https://boards.straightdope.com/u/eldowan)\ **Post date:** [March 22, 2013, 3:10pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/1 "2013-03-22T15:10:32Z")
Straight Dope Classic: [Why is winter the season for colds, flu, etc.? - The Straight Dope](http://www.straightdope.com/columns/read/997/why-is-winter-the-season-for-colds-flu-etc) In this column, Cecil answers a question about the winter months and colds. Recently published in PLOS ONE, this article explores the relationship between humidity levels and influenza viability. > **[PLOS One](https://journals.plos.org/plosone/)** Condensed at sciam here: [Podcasts | Scientific American](http://www.scientificamerican.com/podcast/episode.cfm?id=humidity-levels-explain-us-flu-wint-12-12-27) The gist seems to be: > [@](#): > > … they found that at low humidity, the fake mucus dries up and the virus does just fine. But when the humidity tops 50%, the droplets only partially evaporate, leaving behind a solution that’s too salty for the virus to thrive. > > Interestingly, the virus does well again when the humidity reaches 100 percent, evaporation stops and the salinity of the mucus bath is juuust right. That could explain why the flu prefers to hit the tropics in rainy season. And why you should always keep your nose clean, but moist. > > —Karen Hopkin ---
**Author:** ![Bobbi\_P](https://avatars.discourse-cdn.com/v4/letter/b/ec9cab/32.png) [@Bobbi\_P](https://boards.straightdope.com/u/Bobbi_P)\ **Post date:** [March 22, 2013, 4:49pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/2 "2013-03-22T16:49:50Z")
I believe UCSD scientists confirmed that with lower sun angle, cooler temperatures (so we bundle up), and shorter days, result in a LOT less vitamin D being made by our skin in winter months. Vitamin D is essential for good immune system functioning, so the winter deficiency results in more susceptability to diseases. Dry, heated indoor air and low humidity in extreme cold temperatures also irritate sinuses, making them more susceptible to cold/flu viruses (viri?). Google “UCSD Vitamin D” for more info. You will note that vitamin D also helps prevent cancer by assuring an optimal immune system. ---
**Author:** ![Straight\_Jacket](https://avatars.discourse-cdn.com/v4/letter/s/ad7895/32.png) [@Straight\_Jacket](https://boards.straightdope.com/u/Straight_Jacket)\ **Post date:** [March 24, 2013, 8:59pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/3 "2013-03-24T20:59:18Z")
I think you missed this one: What about the effects that heatings have on drying out the air and thus making peoples mucosa more prone to infections? ---
**Author:** ![JackieLikesVariety](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jackielikesvariety/32/296_2.png) [@JackieLikesVariety](https://boards.straightdope.com/u/JackieLikesVariety)\ **Post date:** [March 24, 2013, 9:16pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/4 "2013-03-24T21:16:12Z")
what about natural light, was this addressed? I think the more light (sun) you get, the better your immune system functions. ---
**Author:** ![Speaker\_for\_the\_Dead](https://avatars.discourse-cdn.com/v4/letter/s/c77e96/32.png) [@Speaker\_for\_the\_Dead](https://boards.straightdope.com/u/Speaker_for_the_Dead)\ **Post date:** [March 24, 2013, 9:22pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/5 "2013-03-24T21:22:51Z")
In colder climates, you also have a lot more people in close proximity with circulated air, since people aren’t going outside, opening windows, etc. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [March 26, 2013, 12:00pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/6 "2013-03-26T12:00:40Z")
Welcome to the Straight Dope, **Straight Jacket** , we’re glad you found us. Since there was already an existing thread on the same topic, I’ve merged your comments into that one. No problem, it just makes life easier for readers. And welcome! ---
**Author:** ![ncnz](https://avatars.discourse-cdn.com/v4/letter/n/ecc23a/32.png) [@ncnz](https://boards.straightdope.com/u/ncnz)\ **Post date:** [March 27, 2013, 12:40pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/7 "2013-03-27T12:40:52Z")
“Cold season” is when the schools are open; it’s the biggest disease-exchange marketplace. Any teacher will tell you the purpose of “winter vacation” is to let the illnesses run their course and die out. ---
**Author:** ![Mr\_Downtown](https://avatars.discourse-cdn.com/v4/letter/m/8e8cbc/32.png) [@Mr\_Downtown](https://boards.straightdope.com/u/Mr_Downtown)\ **Post date:** [March 28, 2013, 4:31am UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/8 "2013-03-28T04:31:17Z")
I’ve always wondered if there are seasonal differences in places like Hawai’i where the weather doesn’t vary much. Don’t Southern Californians get as many colds as the rest of us? ---
**Author:** ![morgensd](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@morgensd](https://boards.straightdope.com/u/morgensd)\ **Post date:** [March 29, 2013, 3:43am UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/9 "2013-03-29T03:43:01Z")
This article from Medscape repeats the “everyone’s indoors” and low humidity hypotheses. Also states that winter months have the highest incidence, but their figure seems to suggest that there’s plenty o’sickness going on year-round. I’m skeptical of the “everyone’s indoors” hypothesis. Most of us spend a good portion of our days indoors in close proximity even in the summer. I can’t paste the pic, but here’s the text and you can see the original at [Upper Respiratory Tract Infection: Practice Essentials, Background, Pathophysiology](http://emedicine.medscape.com/article/302460-overview#a0156) "Although URIs may occur year round, in the United States, most colds occur during fall and winter. Beginning in late August or early September, rates of colds increase over several weeks and remain elevated until March or April.[14] Epidemics and miniepidemics are most common during cold months, with a peak incidence in late winter to early spring. Cold weather means more time spent indoors (eg, at work, home, school) and close exposure to others who may be infected. Humidity may also affect the prevalence of colds, because most viral URI agents thrive in the low humidity characteristic of winter months. Low indoor air moisture may increase friability of the nasal mucosa, increasing a person’s susceptibility to infection. Laryngotracheobronchitis, or croup, occurs in fall and winter. Seasonality does not affect rates of epiglottitis. The figure below illustrates the peak incidences of various agents by season. Rhinoviruses, which account for a substantial percentage of URIs, are most active in spring, summer, and early autumn. Coronaviral URIs manifest primarily in the winter and early spring. Enteroviral URIs are most noticeable in summer and early fall, when other URI pathogens are at a nadir. Adenoviral respiratory infections are most common in the late winter, spring, and early summer, yet they can occur throughout the year. Seasonal variation of selected upper respiratory tract infection pathogens. PIV is parainfluenza virus, RSV is respiratory syncytial virus, MPV is metapneumovirus, and Group A Strept is group A streptococcal disease. Seasonal influenza typically lasts from November until March. In 2009, H1N1 influenza activity was present throughout summer and autumn, overlapping with seasonal influenza. Some parainfluenza viruses (PIVs) have a biennial pattern. Human PIV type 1, the leading cause of croup in children, currently causes autumnal outbreaks in the United States during odd-numbered years. Human PIV type 2 may cause annual or biennial fall outbreaks. Peak activity for human PIV type 3 is during the spring and early summer months; however, the virus may be isolated throughout the year.[13] Human metapneumovirus (hMPV) infection may also occur year round, peaking between December and February." morgensd ---
**Author:** ![AGuy](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AGuy](https://boards.straightdope.com/u/AGuy)\ **Post date:** [April 4, 2013, 9:12pm UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/10 "2013-04-04T21:12:07Z")
The influenza virus has a protective coating that melts at warmer temperatures, making it less able to survive outside of a host, according to the study described here: [Why The Flu Virus Is More Infectious In Cold Winter Temperatures -- ScienceDaily](http://www.sciencedaily.com/releases/2008/03/080330203401.htm) ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [April 5, 2013, 3:28am UTC](https://boards.straightdope.com/t/why-is-winter-the-season-for-colds-flu-etc/653641/11 "2013-04-05T03:28:00Z")
The “everyone is indoors” hypothesis doesn’t seem to work as well this year, at least, around here. We had the warmest winter on record and were out more, yet our flu season was much worse. Of course, there are other factors that could have led to that, but it is the first thing I’ve seen that made me think the hypothesis might be questionable.