I think i read somewhere or was told by a doctor/midwife that your baby will naturally have your immunity for up to 4 months of their life. I didn't BF my DD at all she was strictly FF and she didn't get her first cold until 8 or 9 months i think it was. And not having a stab at BF mothers but a close friend of mine whose DD is 5 months younger than my DD was BF for 6 months and she always had a cold/cough.

So i too don't see how BF is better than FF in regards to immunity... Though my DD has been SICKER by that she doesn't get over it as quickly but i find my friends DD is frequently sick.

I say stick to what you are doing as it will be doing good for your DD but if she doesn't seem to be improving i would see your GP. poor bubba must be awful to be so young and be sick.