When they got sick the first time?

Dd just started preschool, first time at a center although she did a bunch of classes/activities over the summer, so it's not like she's never been sick but it's mostly been colds. I know she will be bringing stuff home, and with how clingy she is I don't know that I can stop the spread. Although she is exposed to a lot now, she didn't get her first cold til almost one!

When is it (not only miserable but) dangerous for a newborn to have a cold? I know many of you had your first in daycare or pre/school when you had your second...did you take any special precautions?