Why do colds and flus happen more in the cold weather if the cold temperature does not actually cause colds or flu infections?

Answer 1

The process is not fully understood, but some of the more current thoughts are that the flu prefers and thrives better in the humidity of colder weather (drier absolute humidity) compared to humidity in other seasons. It is thought that it might be only the humidity level differences that link viral flu and cold infections to the fall and winter seasons, but, as has often been cited, perhaps there is also a component of the closer proximity of people to each other indoors in the cold weather which allows for easier direct transmission of the viruses. Scientific studies have ruled out a role of weather temperature differences. It is proven that you don't get colds from being cold.

Other reasons are sometimes given, for example, one thought is that it is the lowered amount of available sunlight in these seasons and shorter length of time periods that people are outside in the sun that result in lower Vitamin D (the "sunshine vitamin") levels. Deficiency of Vitamin D can impact the immune system negatively, but there is not consensus among the medical community on how much impact this actually may or may not have on infection rates.

Another factor that gets discussed is that there may be an effect from lowered amounts of sunlight, especially ultraviolet sunlight, available in those seasons which helps to de-activate the viruses.

You are correct that it is not the cold temperature that gives us colds, but rather the microorganisms that move from person to person during the "cold and flu season" that make us sick with the flu or a common cold.