I wish my constitution was that good. I don't get sick often - but I do get sick. But lockdown has been good for my immunity so far.
My biggest problem is fungus/fungi of any kind.
Starts with a terrible sneeze and instant epic nose burn . Then I take allergy stuff and leave.
Yes, if your shower has little/lots black/grey/whatever spots I can't visit or very short visit if I take meds. Same if you have a slow water leak in kitchen as mold starts to form with time.
I've had people intentionally cough in my face (they think it's funny) and still seldom that I get flu from that. Bronchitus - last had in 2005, and only cause I was stupid.
I USE to smoke. And even then I was not prone to snot illness.
I am allergic to penicillin and antibiotics, as they are pretty much derived from antibiots. Meaning I can only be treated symptomaticly.
Appendix out, no antibiotics used.
The closest I get to antibiotcs is pure unheated and untreated raw honey. Which I'm happy to eat on a spoon.