B.C.
When my sister and I started school (we stayed home until kindergarten), for about 2 years we came down with everything - and then it settled down to getting sick a few times per year.
When our son started day care at 3 months - for about 2 years he came down with everything - and then it settled down to getting sick a few times per year.
Things improved even more once we'd had our tonsils/adenoids out.
My personal theory is that it takes roughly 2 years of exposure to public germs to train up an immune system.