seasonal viruses

[JeenLeen]

There's a cold and flu season, usually fall/winter. But why?

I can think of two reasons (and one counter-idea), but I reckon neither really capture the full truth. I'm using the terms 'hibernate' and 'migrate' loosely below.
1. The viruses effectively hibernate during the warmer months, as the heat makes it unsafe.
2. The viruses spread across the globe, essentially migrating through the human population as the weather shifts.
3. There really isn't a cold/flu season and you can catch it yearround, and it's around year-round. But human activities are such that fewer people get sick during the warmer months and so it's basically ignored in common parlance.

But #1 doesn't seem to makes sense, and #2 seems odd, especially if there were seasons like this before human populations traveled a lot worldwide.

[Keltest]

Its basically #3. I think the changing temperatures between fall/winter and winter/spring also negatively affect our immune systems in a way that ordinary things start showing up a bit more.

[Thomas Cardew]

The short version is the temperature.

The viruses that cause it neither migrate nor hibernate in a meaningful way. They are detected year round all over the world. However, several of the common virus have a protective layer that breaks down under heat. This lowers the chances of infection as the virus dies more quickly outside of carriers meaning there is less of the virus available to cause infection. But some will always survive in unknowing carriers, favorable environments, or sometimes other species.

During the cold months, the viruses can survive outside the carrier for a longer period of time, lengthening the potential interval and associated risk of infection. Year round research is conducted on cases to predict what viruses are going to be worse that season, the inactivated strains of the most likely varieties are included in that year's flue shot.

[Kaptin Keen]

Conditions can be more or less ideal for the virus' survival and migration. Popularly speaking, when the kids have runny noses, wet clothes, and the heat is turned up, virus has it's heyday, breeding happily and spreading everywhere. As in, autumn/winter.

Conversely in summer, where everything is dry.

I have no idea of the actual science of this - it's just ... the word. It does align nicely with the season for flu, tho.

[Brother Oni]

The simple answer is that influenza likes cold dry air and thus persists longer in winter. This is coupled with that fact the people stay indoors more during winter and hence the increased proximity and breathing each others' air means you're more likely to catch something air/droplet borne.

Note that this does not mean that a hot humid environment is the optimal counter solution as there's plenty of other bugs that like warm moist environments (pathogenic mould for example).

That said, we don't know exactly why it likes cold dry environments - more study is required.

Harvard article.
BBC article.

If you're interested in something more formal: Roles of Humidity and Temperature in Shaping Influenza Seasonality, Lowen and Steel, J Virol. Jul 2014
TL:DR is 5C and sub 40%RH are optimal spreading conditions for influenza. Things get weird in tropical and sub tropical climates.

Its basically #3. I think the changing temperatures between fall/winter and winter/spring also negatively affect our immune systems in a way that ordinary things start showing up a bit more.

You're right, but indirectly. There's less passive Vitamin D production due to less strong sunlight, which has shown to have an effect in boosting the immune system. The decreased light levels can also lead to depression, which also adversely affects your immune system, although this interaction is a bit more chicken-and-egg (depression leading to poor health, or poor health leading to depression).

[Mavezius]

I think the world should get used to the fact that seasonal viruses are normal for the biological organic world in which people live. It's worth taking it in and learning to counteract it effectively.

[JeenLeen]

I think the world should get used to the fact that seasonal viruses are normal for the biological organic world in which people live. It's worth taking it in and learning to counteract it effectively.

Yeah. I used to be very against taking medicine for symptoms (like I'd take an antibiotic, but avoid medicines like benadryl or cough syrup or ibuprofen unless really bad), and I got what was probably a bad sinus infection for weeks each year (mind a little fuzzy, blowing gross stuff out of nose every few minutes to keep it unclogged, etc.), but I just counted it as a stuffy, seasonal nose.

Now that I take some benadryl when symptoms start a bit, I've managed to avoid such bad infections. I reckon it's that it keeps the blockage from getting so strong that bacteria have space to grow and fester.

[Lvl 2 Expert]

There's a behavioral factor as well. Sure, in summer people go to large gatherings to have fun, but when it gets cold they hole up inside, avoid ventilating the place to keep the heat in and spend lots of time close to people.

It's kind of weird how people that don't quite know a lot about viruses often summarize the covid rules as "stay in your home". Going outside is great, and much better for not getting sick in general than staying couped up with your family, just don't go out in a time and place where you'll be forced into licking distance of other people.