Why is winter considered "cold and flu season?"?

I guess this is the time of year when people are most run-down and therefor most susceptible to germs.

Also stuck indoors with sneezing and wheezing kids who don’t wash their hands enough. I am appalled sometimes at restaurants to see young children who are coughing, dripping, hacking and clearly miserable. How far do the droplets from a sneeze travel?

I think that it’s been fairly well established that cold weather (and wet hair in cold weather) does not cause people to get sick. I seem to remember a study from last year that found that some flu organisms survive longer in cold weather, though. And, of course, everyone is inside together exchanging bugs when it’s nasty outside.

One possible cause is that Winter is the arrival of colder weather, and the opening of schools, which prompt people to spend more time indoors and increase the chances that viruses will be spread. Seasonal changes in relative humidity also may affect the prevalence of colds and flus. The most common cold and flu viruses survive better when humidity is low—during the colder months of the year. Cold weather also makes the inside lining of your nose drier and more vulnerable to viral infection.
http://everydayhealth.com/publicsite/index.aspx?puid=04a6e978–319a-40f2-b6ca-3e10ba42e864

Link error, sorry.

http://en.wikipedia.org/wiki/Flu_season
All better now!

@Wizard. Do you live in the US? School starts here just after Labor Day, in early Sept.

I always get sick in the summer. not winter! I have tonsilitis, sinus and ear infection, and I havent slept in two days. I can’t wait for winter. at least I won’t be sick!

Because that is when it is cold out and you get the flu!! :-)

@gailcalled. Sources vary on distance a sneeze travels Between 1–2 meters). It’s a function of obstacles, etc.

@Astrochuck. For a long time, people said that winter “is cold and flu season” because scientists had observed that winter is the time when certain viruses predominate and spread. I will focus my answer on influenza, since it is an easier to study virus than “the cold virus”, which is actually hundreds of different viruses.

Here is a graph of the seasonal incidence of influenza A and B in the United States taken from the CDC website. On the bottom are the weeks of the year, and you can see that influenza predominates in December through March. In the Southern Hemisphere, influenza predominates from May until September (wintertime in the Southern Hemipsphere).

For a long time, people had speculated that the cold weather somehow allowed better survival of the virus, or perhaps better transmission, but direct data was lacking. Recently, a group at NYU showed (note: this technical article is freely available online) for the first time that cold temperatures and low humidity (i.e. dry air) enhance the transmission of influenza (the flu) in a guinea pig model of infection.

This article touched on three potential mechanisms to explain the role of low humidity in enhanced transmission. One, that the mucosa becomes dry (as has been mentioned above). However, owing to the short time frame of their experiments, they did not believe that the mucosa of the animals dried so quickly. Two, that low humidity improves the stability of the virus itself. And lastly, that low humidity favors the formation of smaller droplets of fluid, that can survive longer in the air (compared to wetter, larger droplets) and thus lead to better transmission.

As for the cold air, the authors speculate that “cooling of the nasal mucosa is thought to increase the viscosity of the mucous layer and reduce the frequency of cilia beats [8]. In this way, breathing cold air would slow mucociliary clearance and thereby encourage viral spread within the respiratory tract.”

In summary, influenza has become a seasonal infection in part owing to its relative success and transmissability during the winter months. Now that it has established that seasonal variation, it is here to stay (until global warming kicks in, then all bets are off).

Great answer, Shilolo. Thanks.

cold brings down your immune system so viruses have an easier time attacking

Why would cold bring down your immune system? I think studies have shown that that’s not true.

The sun makes everyone healthier and happier, for example it produces more vitamin D (or does it make it useable by the body, I forget). It also allows us to go out and about more, avoiding being stuck inside together. The body is more germ resistant when it is fit too.

Vitamin D helps you absorb calcium. Not certain if it aids the immune system, though. I always think of vitamin C for that.

OH ok, how about this:

Fruit is out of season in the winter so costs more and therefor people tend to eat less of it. Thus reducing their vitamin C intake.

Actually, vitamin D probably helps the immune system more than vitamin C. There is no evidence to support high doses of vitamin C to prevent or treat a “cold”.

Its when people get it the most. Not summer, fall, or spring.