Question: From the moment we learn to communicate, we always get told, whether by our parents, or our teachers, or by anyone else, to avoid the cold, or to put a jacket on to avoid catching a cold, to dry our hair before we go outside, because we'll get sick otherwise, so on and so forth. I also was diagnosed with a pneumonia last year, while in Switzerland under chilly conditions ($-20^◦\text{C}$). This made me wonder, why does the cold make us sick? It doesn't seem logical to me that a viral infection like the common cold, or a viral/bacterial infection such as a pneumonia is more prevalent when it's colder outside. I always figured it was because our bodies are less 'effective' under cold temperatures, but this seems lacking to me. Can anyone explain?

Answer: 
it does not, really. unless we're talking about things like frostbite or severe hypothermia.

it's a myth that it does.
the virus is more stable in colder air, however.

see more here:

[Study Shows Why the Flu Likes Winter](http://www.nytimes.com/2007/12/05/health/research/05flu.html)

[Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature](http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0030151)

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> Innate responses proved to be comparable between animals housed at 5 °C and 20 °C, suggesting that cold temperature (5 °C) does not impair the innate immune response in this system.
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Most likely the energy to stay warm is not being used to fight infection. Also the virus is likely designed to operate with your respiratory system at a colder temperature more effective that the immune system is at that cooler temperature.