https://astralcodexten.substack.com/p/diseasonality

[epistemic status: conjecture and speculation in something that isn't really my field]

I.

It's still not totally clear why some diseases are seasonal.

Seasonal diseases usually peak in late winter - so around January/February in the Northern Hemisphere and July/August in the Southern. Around the equator, which lacks seasons, they're less predictable and happen throughout the year. The best known seasonal diseases are flu and colds. But viral diarrhea and chickenpox also qualify, as do older mostly-eradicated diseases like measles and diphtheria.

The seasonal flu (source)

The novel coronavirus is probably seasonal-ish, although it's hard to tell since so much stuff keeps happening to make it better (vaccines) or worse (new variants).

The most common theories for disease seasonality are:

1. Pathogens like the cold

2. Pathogens like low humidity

3. People are cramped indoors during the winter

4. People have low vitamin D during the winter, and vitamin D helps fight pathogens

None of these are really satisfactory on their own.

Cold and humidity are definitely important - scientists can make flu spread faster or slower in guinea pigs just by altering the temperature and humidity of their cages. But it can't just be cold and humidity. But if it was just cold, you would expect flu to track temperature instead of seasonality. Alaska is colder in the summer than Florida in the winter, so you might expect more summer flu in Alaska than winter flu in Florida. But Alaska and Florida both have lots of flu in the winter and little flu in the summer.

(if it was just humidity, same argument, but change the place names to Arizona and Florida.)