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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?

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Generally, I presume it is caused by a decrease the enzymes in the body's rate of activity, causing slowing of vital bodily functions like the immune system. There are probably a plethora of specifics to add to this. –  Alyosha Dec 8 '12 at 23:25
    
I agree with Alyosha above, I don't think that coldness is a cause of disease as much as a contributing factor. And cold air is also more dry, causing your tracheae to dry out, becoming more susceptible to any potential virus and bacteria... –  Zewz Dec 9 '12 at 1:26
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When the body gets cold, the body diverts energy away from the immune system to the effort to stay warm. –  Gabriel Fair Dec 10 '12 at 15:53
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I'd prefer answers to this questions under 'answers', not under comments. –  ZafarS Dec 10 '12 at 21:10
    
I've looked around a bit and this is a hard one - its clear there is a corelation but its hard to find an explanation. –  shigeta Dec 14 '12 at 0:31
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4 Answers

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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

Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature

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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But a virus must be considerably more stable in colder air, since 'catching a cold' from the cold is very easy, and I have never caught a cold in the summer. Is it that considerable a difference? –  ZafarS Dec 16 '12 at 20:15
    
The virus may also come from prolonged exposure to rain. I would argue (unfortunately without substantiation) that it is the core body temperature, or the temperature differential that is the factor. –  Everyone Dec 17 '12 at 15:44
    
"a virus must be considerably more stable in colder air" -- yes, it is. see the links above. low humidity also plays a role. –  kto Dec 17 '12 at 21:35
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I think this site best answers the question:

Cold Weather Can Make You Sick

Here is a part:

"Professor Eccles explained this effect by saying that our bodies restrict blood flow to the extremities when we get cold to help conserve body heat for the torso and brain, which really need to be warm. Cutting off the blood flow reduces the supply of white blood cells which are the immune system’s primary weapon against germs.

While his explanation makes sense, there may be a more general effect at work. The human body is a machine that accepts fuel in the form of food, and uses that fuel’s energy to keep us warm and to power our immune systems, muscles and brains. However, in frigid conditions our bodies have probably evolved to say “who cares if I might get sick a week later when I’m going to die of hypothermia in half an hour?”"

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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.

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This is what I thought but how can for instance wet shoes/cold feet increase the risk of getting the common cold virus? Feet are so far from the respiratory system.. –  daniel.sedlacek Feb 28 at 12:07
    
It is still a shock for the whole system. –  Fabio Feb 28 at 19:35
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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.

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It would be a stronger answer if you included some references to back up your statements. –  jonsca Dec 14 '12 at 0:18
    
"O" as in observation or "O" as in opinion? –  Chirag Dec 15 '12 at 18:50
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