eli5 If children’s (0 to 5) immune system is weaker or still growing, then why is it ok to let them get sick to build their immunity?

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I’m hearing lot of moms say that it’s ok to let children play in dirt, attend daycare with sick kids, not sanitize the living spaces too much, put dirty stuff in their mouths because they can only build their immune strength by getting sick first(?!).
I have read the hygiene hypothesis paper and many research papers that have disputed that paper aswell so I’m just curious to learn the science behind the idea ‘let them get sick first’.

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Things that are not especially dangerous in themselves likely give the immune system a work out that prevents it in some way turning on its ‘host’. That doenst mean that you want them to get sick with anything dangerous – that’s what we have vaccinations for. I guess its more like it allows the system to calibrate risks and responses properly?

Please note that I’m not condoning or disapproving of this practice, just explaining the thought behind it.

There are a number of common illnessess/things we can get an immunity to. If you expose a child to them slowly, over time, they are likely to gain immunity to a number of them before they go to school, at which point they’re likely to be exposed to almost all of them in rapid succession, so the thought is that, if you have some immunity, you’ll not get as sick as often once you’re in school. You “spread out” the ~~inievitable~~ *inevitable* illness.

It makes sense.

On the other hand, we were pretty much constantly sick when our kids were young because there’s always another variant of something going around, so your immunity isn’t always all that helpful.

So it also makes sense to avoid them getting sick whenever you can.

I don’t know what the best course of action is. I just am trying to present the thinking behind this.

You are right that it does sound crazy and it is not recommended health practices. There are some diseases which does not affect kids as much as adults and before we had vaccines people would try to get their kids sick with these diseases before it would be potentially fatal. However other diseases are much more harmful to kids then adults so this was only done for a few specific diseases and the practice mostly stopped once vaccines became available.

There are also studies showing that kids growing up in very clean environments have a greater risk of developing auto immune diseases and asthma. This might have something to do with the immune system not being exposed to harmless chemicals and therefore reacting too strongly once exposed to them. However this should not be taken as advice to intentionally expose kids to dirt as the risk of harmful infections are much greater then the risk of these illnesses. And while playing in the dirt is not that risky to kids unless there is a bacterial outbreak in the area intentionally exposing your kids to other sick kids will shorten their life expectancy.

There are certain illnesses that have been around for centuries where it’s better to get it younger. Like if you get it below age 7 you’ll definitely be fine, and if you get it after 15 you have a much higher chance of dying. And you can only get it once so it’s better to get it younger.

People just sort of assume that applies to everything when it really doesn’t.

Most of the bacteria we encounter in the world are harmless, some are beneficial or necessary (especially in the gut), and a few are harmful. The young immune system knows how to get rid of invaders but not which ones to get rid of. Early exposure to a wide range of them (like those found in dirt or in a normal living space) let’s the immune system figure out that it can and should leave some of them alone.