The oral polio vaccine contains a live virus that is easily spread to others via stools. That is why the US phased out this type of vaccine several years ago. It is speculated that this girl recently received the oral polio vaccine in another country.
I saw it reported elsewhere (I cannot recall where at the moment) that this was identified as the source of the polio infection in the other 4.
Well, if you don't poop on other people, what's the problem with that?