I only ever get none contagious illness really (ear infections)
Norovirus did the rounds in my house, with two young kids at the time, DH and both of them were ill...bounced off me.
I did get Covid in February, but rarely get colds etc
I'd say it's exposure over lifetime. Played outside all the time as a kid, the lifestyle of out after breakfast and home with the street lamps kind.
My house is kept cleaning, but hygiene could be better (e.g. I'll use the same mug for tea all day, don't wash bath towels more than once a week and we share them, I don't use seperate chopping boards - though I do cut raw meat last)
More interestingly is that I can count on one hand how many antibiotics my kids have had between them, and my eldest is nearly 13.
They both ate a slug when they were kids so I'm saying it's that....