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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Cold weather makes your body use more energy for keeping warm and less energy for other activities. This is made at various levels: 1) modifications of the diameter of arteries changes the blood supply in specific regions
2) In the electron transport chain, that is a metabolic pathway involved in producing energy after glycolis and Krebs cycle there is a disjunction between the electron transport chain and the oxidative phosphorylation caused by the production of thermogenin. This causes a minor production of ATP (energy available for metabolic processes) and dissipation of energy as heat.
Moreover enzymes need a specific environment to work (ph and temperature). A cold weather can decrease the temperature of the body and reduce enzyme activity. When you are sick your body can use fever to stimulate enzymes and power immunitary system activity.
Moreover cold weather causes a reduction of the movements of the cilia of the trachea, so dust and bacteria can enter into the respiratory system more easily. These are just a few examples. There may be others. I'm available for further discussion.