Children up to six months are protected by antibodies from their mother. After that there is a window until the recommended age of one year to get their first measles shot. It is possible to get the first shot at nine months, but then you need two boosters instead of one to get high immunity, and you still have that three month window of susceptibility.

Then there are those who can't get immunized such as children on chemotherapy.

It isn't just the children of parents that chose not to immunize that are at risk. That is why all the hullabaloo.