Great question!

A number of explanations for the timing of the flu seasons have been suggested, and they tend to broadly align with your own speculations:

* People tend to spend more time indoors in the winter, thus facilitating transmission between individuals in closer quarters.
* There is less light in the winter, and specifically UV light. UV light can easily damage viruses when outside of the body, making transmission potentially easier than during the summer. Exposure to ultraviolet-B also affects vitamin D production in the human body, which in turn may affect the immune system.
* Viruses are better preserved on surfaces, when it's cold.
* According to some fairly recent research (Huang, Di, et al. "Cold exposure impairs extracellular vesicle swarm–mediated nasal antiviral immunity." Journal of Allergy and Clinical Immunology 151.2 (2023): 509-525.), a drop of just 5°C in ambient temperature can halve the number of a certain type of antiviral cells.
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence