DS got sick a LOT for the first two years. Quite badly too with lots of bad chest infections/bronchiolotis, asthma, seriously bad viruses. I used to lay with him all night long just to hear him breath ( wheeze) He was BF exclusively only 4 months. I don't think it has anything to do with not being BF tbh.
By his third winter he was heaps stronger and barely got sick - yet to see how this winter goes and praying he's the same. So maybe those two years of absolute hell did him wonders.