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: 
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.
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.