B.C.
When my sister and I started school (we stayed home until then), we got sick with just about everything that was going around, then settled down to one or two colds a year.
My son started day care at 3 months old and for the first 2 years he came down with everything that was going around, then settled down to one or two colds a year.
My theory is: it takes roughly 2 years of getting sick a whole lot before the immune system has been programmed well enough to fight off most bugs.
We ate healthy, did probiotics and vitamins and it made no difference.
Just hang in there.
It will get better sooner or later.