I wouldn't be the least surprised if Vaccinations have something to do with it. My sister in law had a baby die of SIDS (that was the cause given). I know this woman and she loved that baby more than herself- so it was something other than anything she did... That much I know. Vaccines sounds plausible.

My first wife had a theory that babies somehow forget how to breathe (don't ask me) For the first year we always kept the crib right next to our bed so they could subconsciously hear us. I'm always suspect of abuse in foster care, probably because of my own experience.