Infectious period typically begins a day prior to onset of symptoms through day five or so of illness. Thus, there are people who may exhibit no outward flu symptoms that can be contagious too.
Best to practice common sense cleanliness our mothers taught us...frequent handwashing; discourage children (and adults who act like children) from biting fingernails, picking the nose, rubbing eyes, etc.
Is there any way to predict how big this will ultimately be or do we have to wait and see?
Also would not common sense indicate travel warnings from the state dept. for travel to at least Mexico City if not all of Mexico?