[donna]

A goal of the recommendation is to prevent teens and adults from spreading the disease to infants, although there's not good evidence this "herd immunity" approach has worked so far.

It's just my personal opinion:
1) Mothers used to keep their infants at home away from germs until at least 6-months. Now they abandon infants to daycare.
2) Illegal aliens are spreading diseases.

[LaRueLaDue]

1) Mothers used to keep their infants at home away from germs until at least 6-months. Now they abandon infants to daycare. 2) Illegal aliens are spreading diseases.

I think both factor in, but item (2) is the bigger problem. They are bring in the diseases, in addition to spreading them around.