I wondered if I could pass on some information that I learned from hard won experience in case it helps other parents?
My DS was terriby susceptible to viruses when he was a toddler and in his early school years. He lost an awful lot of school attendance and had just terrible illnesses all the time.
Anyway, we found out in the end that he was short of iron.
Once I figured that out, I started giving him measured amounts of fortified breakfast cereal, to get him the exact RDA of iron every day, and he became fabulously virus-proof and stopped getting ill (touches wood). It really was so much better and he went right up to 100% attendance.
So if your kid suffers with a lot of viruses, it might be worth checking their diet to make sure they have enough iron and B vitamins. It's worth remembering that all wheat flour is fortified with iron and some B vitamins, as a legal requirement. So if you are counting, you need to count stuff like bread, and if your child is off wheat then you need to count extra carefully because a whole lot will be missing.
I should admit at this point that he did get covid last year and had a really bad cough for a month, but other than that, he is good.
Hope that helps some folks out there.