brian zikmun-fisher: is it okay to question vaccines? of course it is. it's a start of a conversation that says, "this is something worthy of being concerned about." narrator: decision scientist brian zikmund-fisher studies how we think about risk. he's based at the university of michigan, where, in this hall in 1955, the first polio vaccine was announced. zikmun-fisher: 50 years ago you asked any parent, any grandparent about polio, about measles, about pertussis and they knew cases. it was part of their daily existence. what's changed now is that we've done such a good job of vaccinating most-- not all, most-- people in our communities, that they are rare. i have never seen it. why should i be concerned about this? paul offit: the history of vaccines is clear. if you start to decrease vaccine immunization rates, you start to see the diseases re-emerge. it's a history that we don't seem to learn from. narrator: paul offit is an infectious disease expert. he helped invent a vaccine to protect against rotavirus disease, which kills hundreds of thousands of children worldwide. as a pediatrician, he's witnessed firsthand children dying from preventable diseases. offit: i guess we all have our