yuliya patsay: there's just so much information. i don't know who to ask. who am i supposed to trust? narrator: in a world of often conflicting information, parents are seeking what's best for their families while doctors worry about losing lives. paul offit: your job is to try and save children's lives, and when you stand helplessly by and watch them die, it gets to you. amy middleman: we ask a lot of parents, we ask them to trust that we are recommending the best thing for their children. that's a big deal. narrator: the science behind vaccines-- why they work, how they work, and how we decide to vaccinate or not. it's not about saying, "oh, my gosh, i'm afraid" and now that's the end of the conversation. no, that's just the start of the conversation. narrator: "vaccines: calling the shots." right now on nova. major funding for nova is provided by the following: boeing-- where the drive to build something better inspires us every day. supportinova and promoting public understanding of science. and by: and by contributions to your pbs station from: additio