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Most bacteria that infect warm blooded animals are optimal at 37 degrees C. This is why bacterial cultures are grown in incubators at 37C. This is also believed to be the reason that one develops a fever when infected. The increase in temperature slows the maximal growth of the bacteria.

As to viruses. It isn't the temperature, Its the humidity. In moderate temperatures, during cold weather, people are huddled together indoors. The lowered humidity of indoor heating systems dry out the mucus membranes that contain antibodies white blood cells and other factors that inhibit viral penetration. the viscous fluid layer is also a mechanical barrier to viral penetration. But as humans are in such close proximity, the bodily fluids that contain the infective virus don't dry out soon enough to have any effect on the infectivity of the virus and the dried membranes are a compromised barrier. Hence increased infectivity and rapid spread.
In extremely cold invironments, the relative humidity is so very low as is the population density that the potentially infective virus dries out very quickly. Cold dry air plays two roles in inactivating the virus. Both by drying the virus as well as altering the virus' three dimensional structure necessary for infectivity.

Cold and drying only inactivate the virus temporarily. Were the virus to be rehydrated, it would be again infective. This is how vaccines are maintained until it is time for use. They are dehydrated, chilled and placed in a vacuum. Still they are immediately infective when reconstituted with water or saline solutions.
In extreme Northern climes, eventually, oxidation and radiation inactivate the vast majority of viral particles before they ever have the correct set of factors that permit infectivity.
It is my understanding that one of the small advantages of being stationed at McMurdo Station(South Pole) is that since no one with a cold is allowed anywhere near the station, you can at least be assured that you will never catch cold during your entire stay.
Many viruses- rabies,HIV for example can be easily deactivated by modest changes in temperature, humidity or exposure to ultraviolet light. This explains why certain viruses can only be spread under optimal conditions.
Still, there are many reported cases of individuals developing rabies not from a bite or scratch but from spending extended amounts of time in bat inhabited caves. Its dark, the humidity is high. bats excrete the virus in their urine. The virus stays suspended in the urine and is protected in the moist dark environment. The cave explorer inhales the virus and is infected.
I used to work in infectious disease control.
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