I have 3 children: 11, 8, 8.
They have had chickenpox and this winter, for the first time in their lives, a vomiting bug.
Other than that, none of the three of them has ever had a day off school for illness, never more than mildly stuffed up, and none of the three has ever had a GP appt in their whole lives except standard newborn tests/ vaccines.
I suspect partially luck/ genes but....
We are clean but zero bleach or antibac (I personally think over-use of these harms microbiome).
They eat a lot of fresh food, so non processed stuff. They do like UPFs but a typical meal would include a whole raw carrot, raw spinach leaves etc.
They do not have any sugary drinks (for example we do not have juice in, ever). We do not usually have biscuits, sweets or chocolate in. I do not have a scientific explanation for this, but when I was younger, sometimes if I ate a lot of biscuits I would be more likely to pick up a bug later.
They play outside and in dirt a lot, digging a "pond" in the back garden etc.
Obviously, they wash their hands. But I do think they get exposure to more germs than most kids and eat more fresh real stuff. I cannot prove that plays a role in immunity but they are noticeably less ill than other kids we know.