[cheryl j (OP)]

This article in Science Daily claims scientists have explained the winter - flu season connection, but a lot of it didnt make sense to me. It claims influenza survives better because the dry winter air sucks the moisture out of droplets. But I always thought respiratory viruses like moisture - that's what makes them  respiratory viruses in the first place. Also, do people in year round warm climates not have a flu season?

My own theory is that people are indoors more in the winter, and schools are in session. Schools have a large number of individuals who have never encountered the hundreds of cold viruses that exist and pass them easily. Ask any parent of a kindergartner or someone in their first year of teaching.

http://www.sciencedaily.com/videos/2008/0103-does_winter_cause_the_flu

[CliffordK]

I'm having troubles with your link.  Is it broken?

I would think the theory that "dry" weather is better for the cold and flu would be problematic.

In Oregon, the relative humidity is much higher in the winter, and lower in the summer (that is why winter mold is such a problem).
Elsewhere in the USA, there is high humidity in the summer, and low in the winter.  Especially with indoor heating.  The cold & flu season here seems independent of the humidity.

Of course, it is all relative humidity, but the "dryness" is determined by the relative humidity.

Yet, despite the wet winters and dry summers, our cold & flu season is in the winter.

Drying out the viruses also sounds like it would be bad for many viruses, so hot/dry weather is likely to be hard on them.  But, I don't know about hot/humid weather. 

Colds seem to be somewhat independent of central heating/cooling vs wall electric heating, or other non circulating heating systems.

Can the viruses live longer at 30-40°F (0 to 10°C) than at 90°F (30°C)?

Of course, some people keep their indoor thermostats quite high in the winter and low in the summer.  Sometimes at the same temperature inside year-around.

You are right that packing school kids together would be a risk factor.  Even for those of us without school kids, the schools amplify the diseases that are in the community. 

It is my belief that cold weather causes stress that is also a risk factor for colds. 

[Lab Rat]

Your theory is correct. 
More people stay inside during the winter .  That means that there are more pathogens brought inside a smaller space, opposed to outdoors, during the winter.  This, of course, heightens the probability of contracting a pathogen, especially airborne ones. 

[Bored chemist]

Also, sunlight "kills" viruses.

[virologist85]

Hi Cheryl,

I'm a virologist, so I can give you some input on your question.

Influenza is a virus and viruses are not dead or alive. It's a respiratory virus because it replicates best in the cells in your nasal cavities and lungs and is spread by the aersol when you sneeze, blow your nose, etc. This leads us to your questions about humidity. Flu can spread from the aersol that is made when you sneeze or cough. Humidity in the air weighs that aersol down (think of traveling through a pool versus traveling through dry air as an exaggerated example). When it's weighed down the aersol doesn't travel as far so it may only go a couple feet instead of 10 or 15 (you'll only feel it if it's closest to you but the smaller aersol droplets can go very far). In the winter the air is dryer so the aersol (and therefore the virus contained in it) can travel further and have the ability to infect more people.

You're also right in saying closer proximity to other people inside plays a role--greater risk of contact with someone that's infected.

Hope that helps. If you're interested in viruses, I'd encourage you to check out virologyblog.ws ; the writer is a professor that is very good at explaining these things and commenting on virus topics in the news.

[yellowcat]

Would the colder conditions and lower level of UV allow the virus particles to remain infectious for longer making fomite transmission more likely.

[RD]

... You're also right in saying closer proximity to other people inside plays a role--greater risk of contact with someone that's infected.

so when the state-funded NHS instructs all the coffin dodgers OAPs receiving a state-pension to gather together in the town-hall for their annual flu shot there may be an ulterior motive. []