I think what MadB explains it perfectly. My DS1 was fully FF from 4 weeks too but never got sick till daycare either but he didn't have other influences in his life that could bring in the nasties, where as DS2 not an overly sick child but would get what was going around cos DS1 by this stage was in school etc. I think you also need to factor in the time of year and whether you do MG or things like that where other influences come into play. There is absolutely no question BM is much better for a bub.

Kazzo my Aunt used to do the bongo's on my nephew's back when his asthma was bad, it really helped in loosening up the mucus on the chest.