Why do people get the cold and flu in the winter time?

People say flu season is in the winter from October to April. They say it not the cold weather causing it but people indoors more but I’m confuse because in the sun belt cities like Phoenix and Miami in the summer the AC is on and they are indoors more yet they get cold and flu in the winter time there with the AC off in the winter there and not the summer.

Also the window is open and they are out doors more in the winter time there.

So what is causing the cold and flu to spread so much in the winter time?

Other warm tropical countries where it is hot all year round get the cold and flu there.

IIRC, one factor is that colder, drier air helps the virus survive longer - heat and humidity impede it.

edit: yup

https://samaritanmedicalcare.com/why-the-flu-is-more-common-in-winter-the-science-behind-the-surge/

The virus has a protective lipid layer that becomes more stable in cold, dry environments. When temperatures drop, this coating hardens, allowing the virus to survive longer outside the body. This makes transmission easier during the winter months, as the virus remains infectious for extended periods on surfaces and in the air. 

Moreover, the lower humidity common during winter months further aids the spread of influenza. In dry air, respiratory droplets containing the virus can travel farther, increasing the likelihood of transmission

9 hours ago, swansont said:

IIRC, one factor is that colder, drier air helps the virus survive longer - heat and humidity impede it.

edit: yup

https://samaritanmedicalcare.com/why-the-flu-is-more-common-in-winter-the-science-behind-the-surge/

The virus has a protective lipid layer that becomes more stable in cold, dry environments. When temperatures drop, this coating hardens, allowing the virus to survive longer outside the body. This makes transmission easier during the winter months, as the virus remains infectious for extended periods on surfaces and in the air. 

Moreover, the lower humidity common during winter months further aids the spread of influenza. In dry air, respiratory droplets containing the virus can travel farther, increasing the likelihood of transmission

I presume the latter point is that the droplets evaporate, leaving a tiny dry nucleus of virus particles, which can drift in the air more easily.

There is also, at least in climates like that of the UK, the issue of less sunlight in winter months (shorter days, more cloud cover) which reduces the sterilising effect of UV sunlight.

I think we all became far more aware of these factors during the Covid pandemic. Certainly I knew nothing of the reasons until then.

As for A/C, my experience in Dubai in the 1980s was that people caught colds in summer, presumably as a result of having all the windows shut and breathing recirculated air. There was also a theory about the shock to the body of moving between an external temperature of 40C with 95% humidity and a low humidity indoor temperature of <20C. But I have never seen that substantiated.

10 hours ago, Moon99 said:

Other warm tropical countries where it is hot all year round get the cold and flu there.

We, in a warm tropical country, have rather a "high season", when herds of tourists from cold countries come in carrying the virus with them.

3 hours ago, Genady said:

We, in a warm tropical country, have rather a "high season", when herds of tourists from cold countries come in carrying the virus with them.

In that regard it should be noted that flu seasons are not universal and different patterns can provide overlapping contributions. For example, in tropical regions seasonality of flu is often less pronounced, but might be driven by other areas as Grenady said. Similarly, areas with high levels of travel can have similar flu patterns, even if weather patterns are differently. In either case the main driver is the increase of infected people driving further transmissions.

So even if absolute humidity might not be a contributing factor in one area, travel eventually leads to a global spread each year. There are also local influences that have been described in literature. Examples include waning immunity from prior exposure as well mutation rates (and presence or absence of non-human reservoirs), but the associations tend to be weaker, especially on more global scales.

Not so confident in the cold/harden envelope concept. Couldn't find support in literature and infective expsure is likely an indoor phenomenon where indoor temperature would be controlled.

https://journals.asm.org/doi/full/10.1128/aem.00551-18

above article includes this graph - temperature does impact infectivity(a flu virus surrogate) esp. in mod to higher RH. Recall it is commonly assumed folks are exposed to lower hummidities in indoor auir in winter.

t

4 hours ago, PhilGeis said:

Not so confident in the cold/harden envelope concept. Couldn't find support in literature and infective expsure is likely an indoor phenomenon where indoor temperature would be controlled.

https://journals.asm.org/doi/full/10.1128/aem.00551-18

above article includes this graph - temperature does impact infectivity(a flu virus surrogate) esp. in mod to higher RH. Recall it is commonly assumed folks are exposed to lower hummidities in indoor auir in winter.

t

IIRC epidemiological lit also shows a stronger correlation with humidity than temp. In medical lit (or at least those that I remember) tend to focus on mucosal damages, rather than viral stability, though.

Edit: I think one of the papers I read on that matter was this one:https://www.pnas.org/doi/abs/10.1073/pnas.1607747113

That is really interesting because I hear tropical countries there flu season is the summer time but in the US in the sun belt cities it is the winter time.

Well cities like Los Angeles, Houston and Miami and much of Florida it is in the winter time not the summer like the tropical countries.

I don’t know why that is the case?

Edited Monday at 03:39 PM2 days by Moon99

20 minutes ago, Moon99 said:

That is really interesting because I hear tropical countries there flu season is the summer time

Where did you hear this? From a credible source?

https://www.psu.edu/news/research/story/influenza-shows-no-seasonality-tropics-posing-challenges-health-care

“new research led by Penn State has found little evidence of a repeatable pattern in influenza cases in Vietnam. The findings suggest that influenza is likely unpredictable throughout the tropics”

https://respiratory-therapy.com/disorders-diseases/infectious-diseases/influenza/influenza-tropics-shows-variable-seasonality/

“The authors found that flu activity patterns in the tropics and subtropics were much more complex than in the temperate northern and southern hemispheres. They were able to discern patterns in influenza activity for 70 countries and found most of these had one or two distinct peaks per year. Countries nearest to the equator often had year-round flu activity.”

(emphasis added)

Also in the abstract in the article above:

In temperate countries, influenza outbreaks are well correlated to seasonal changes in temperature and absolute humidity. However, tropical countries have much weaker annual climate cycles, and outbreaks show less seasonality and are more difficult to explain with environmental correlations. Here, we use convergent cross mapping, a robust test for causality that does not require correlation, to test alternative hypotheses about the global environmental drivers of influenza outbreaks from country-level epidemic time series. By moving beyond correlation, we show that despite the apparent differences in outbreak patterns between temperate and tropical countries, absolute humidity and, to a lesser extent, temperature drive influenza outbreaks globally

The flu season in the (sub)tropical areas in the US are likely driven by travel from the temperatre regions (as discussed earlier in the thread).