Simplified analogies suck... there is more and more evidence that (some) virus infections can have a damaging accumulating effect (it depends though and the biggest factor seems to be the individual). By your simplified analogy it would seem logical to expose oneself to any virus as often as someone does not "overexercise" the muscle, which is really not recommended!!! It doesn't make you stronger, it can weaken and permanetly damage you! Some things are lethal, some things are damaging, hyperhygiene especially in childhood is not good, but still hygiene is a profound reason our civilization does better in many regards.. it is a tight balance everyone needs to decide for his own (and to his own conditions).
Please don't propagate that our immune system is a muscle we need to train (in that simplified way).
Your immune system doesn't "take breaks", if it would you'd just die pretty quickly. The immune systems is constantly confronted to the germs in your environment and doesn't have to be actively fighting an identified disease to keep working properly. There is no such thing as "immunity debt".
The immune system seems to work in somewhat the opposite way that you suggest. There is growing evidence to support the hygiene hypothesis. Living in overly sterile environments can cause the immune system to go haywire and start attacking the body's own tissues (autoimmune disease). We co-evolved with germs, and need a constant exposure to some level of germs to stay healthy.
Your immune system actually never takes a break: it's continually working intensely, that's why/when you're not ill most of the time.
Case in point: your body _constantly_ gets "agressions" from bacterias, viruses, germs from the air your breathe, the things/people you touch, the scratches you make, what you eat.
When you get symptoms (a flu, a cold, whatever disease), it's because your immune system gets overwhelmed, is working even harder, has a hard time to keep up.
When you get exhausted for other reasons (not enough sleep, too much exercise, stress, etc.) your body has less energy for your immune system to work properly. This explains why you easily catch an infection you wouldn't when you're not exhausted.
It's more like it "got a break" because people were quarantines, so now you haven't had the flu for 3 years it will be less likely to handle it than if you had it last year. Flu spreads primarily via bodily fluids (door knobs, touchy kids, etc) and people are now more likely to get it and a huge swathe of the population hasn't had it in a while, hence this year will likely be a bad one, along with more people who haven't had flu in a while.
Getting the flu is not a requirement for well-being. I've spent several years not getting it, then getting it once, then again not for several years.
It's a matter of preventing it (washing hands often, masking in crowded areas, ventilating rooms properly) and helping our body system to be rested/strong (getting enough sleep/rest, dressing warm, drinking enough). And of our own conditions. And luck.
Yes, immune system memory is at play, but even with the exact same virus, getting the flux in year 1 doesn't mean you'll not get it in year 2 if your body is unwell already.
Moreover, getting the flu (or any infection really) is not a small thing: it can break you down for days, weeks, months and even kill you. Best to avoid it when you can, and be happy when you avoided it.
The pseudo darwinist thing of "getting infected trains your immune system / makes you stronger" or "what doesn't kill you makes you stronger" is bs on the other hand.
The claim above was "Our immune systems can't just 'take a break'... Its a muscle, you don't use it; it atrophies."
Polio disproves that conclusively.
Your explanation agrees with me here; describing it as a problem with the immune system is inaccurate. Relatively rapid immune escape is a feature of certain viruses.
Polio is slow, and it has to go through the blood to cause severe disease. RSV and COVID-19 (or really SARS-CoV-2) don't. IPV can't stop transmission, and OPV has dubious long term protection from spread.
