connected every day in a thousand unseen ways. but sometimes these connections can pose an invisible threat... if the object we touch or the air we share carries a dangerous germ. simon fensterszaub: you don't have to cough, you just have to breathe. it's the worst kind of contagion; it's airborne. narrator: diseases largely unseen for a generation are returning. fensterszaub: i'm, like, "this can't be. how does a kid have measles?" this is new york! we don't see measles in new york. jane zucker: it's astounding to me that you can have people dying of measles in the developed world. narrator: today, children are getting sick and dying from preventable diseases

as nervous parents skip their children's shots. alison singer: you are injecting a substance into your child, so i think it's very natural to wonder whether that substance might actually be doing harm. 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: additional funding is provided by millicent bell through: narrator: happy, healthy children.

it's what every parent wants. and in this era of modern medicine, it's what most parents expect. many dangerous diseases have all but disappeared, thanks, in good part, to vaccination. in the u.s., more than 90% of parents vaccinate their children, and most follow the recommended schedule. that's up to 28 immunizations in the first two years of life to protect against 14 different diseases. but today, powerful concerns are in circulation. woman: i'm concerned about how many vaccines we have to give our children at once. woman: so, i'm kind of debating whether... i will do them, but i'm debating the age, when should i have them done.

woman: there's just so much information there, and i don't know who to ask. there's no such thing as an unbiased source. narrator: at least ten percent of parents delay or skip some shots. around one percent don't vaccinate at all. and in some places it's far higher. what's driving some people away from vaccines? and what are the consequences for those who vaccinate and those who don't? (siren wailing) (child coughing) are you okay? (monitor beeping, child continues coughing) narrator: osman chandab is seven weeks old. he was due to be vaccinated against whooping cough in the next week, but the germ has gotten to him first.

his mother brought him into the hospital two days ago. what started as a runny nose and slight cough has become frightening episodes where he's struggling to breathe. whooping cgh-- or pertussis, as it's formally known-- can be life-threatening for babies. daryl efron: whooping cough is caused by a really nasty bacteria called bordetella pertussis, and it produces a toxin and it attacks the airways and causes a really nasty bronchitis. and they get really sticky, thick phlegm that they can't shift, and they try to, and that causes them to cough. (baby coughing) and the cough needs to be really, really vigorous, but they often... it's not very effective and then sometimes they stop breathing. and if they stop breathing for long enough, they go red in the face at first and then sometimes, if it's an extended period of not breathing, they can go blue, and that can be very, very frightening.

(baby coughing) narrator: half of all infants younger than one will be hospitalized. about one in 100 dies. (monitor beeping) antibiotics can reduce the chance of infecting others, but there is no cure. osman's tiny body must fight the illness. a generation ago, whooping cough was rarely seen in developed countries. now it's back. in 2012 there were nearly 50,000 cases in the u.s. and 20 deaths. babies too young to get the pertussis shot may have no protection. and because the vaccine's effectiveness can wear off after a few years,

older children and adults without boosters are also vulnerable. since the beginning of our young century, there have been outbreaks of vaccine-preventable diseases, flaring up like so many wildfires. in 2011 there were over 30,000 measles cases across europe. although the disease is often mild, thousands had serious complications; eight died. in 2012, the outbreak continued in the united kingdom. in 2013, it made the leap to the united states. (cars honking) a young american traveled from london to brooklyn, infected with the virus. he returned home to his tight-knit orthodox community where most people vaccinate, but he came from a group of families strongly opposed

to vaccines. measles quickly spread through unvaccinated families living in the same building and then out into the community. children were hardest hit, especially those too young to be vaccinated. simon fensterszaub: the first case that i saw was a 12-month-old girl that came into the office with mom because of a rash. and i'm looking at this child, examining this child, and, like, "this can't be. this child actually has measles." i've never actually seen measles, but, uh, i've seen them in textbooks, never in real life. so i was a little taken aback. three or four days later, perhaps, we had another case and then the next day, another case and then another one. and it just, it started getting overwhelming. like, wait a second.

narrator: the department of health tried to stop the spread. jane zucker: the thing about measles is it's droplet-spread. so, for example, you know, your respiratory secretions, if someone coughs. but it's also airborne. so the virus can sort of hang out in the air for up to two hours. narrator: one case was traced back to an infected person taking the elevator up to an apartment, shedding the virus along the way. the measles virus usually infects the cells of the throat and lungs, but it can also survive in the air and linger on objects. fensterszaub: you don't have to cough, you just have to breathe. it's the worst kind of contagion-- it's airborne. narrator: 90% of people who are exposed to the virus and don't have immunity get the disease. in this instance, two hours later,

another unvaccinated person entered the same elevator. a week or so later, he too became ill. zucker: it was a very brief encounter. you know, seconds in an elevator. it just speaks to how infectious measles is. it will find those people who are unprotected in a community. narrator: in three months, more than 3,500 people were exposed to the measles virus in the brooklyn area. 58 were infected, including two pregnant women. one miscarried. all were confirmed as unvaccinated at the time of infection. measles was declared eliminated back in 2000, and so this was the largest outbreak since elimination. fensterszaub: it's like telling me, "look, we found smallpox." smallpox is eradicated. i mean, no one sees measles. who sees measles? yeah, you'll see measles in third world countries,

in other countries. this is new york! we don't see measles in new york. narrator: around the world, pockets of low vaccination are appearing, often in affluent, mainstream communities. anthropologist heidi larson studies why people do or don't place trust in vaccines. larson: there isn't just a world polarized between pro- and anti-vaccine populations. there's 80-plus percent in the middle who it's either just a social norm, it's good for our children and for our public health, others that are starting to question, and a certain amount of questioning is healthy. and then there's some that are starting to get more anxious and are vulnerable to tipping into becoming against vaccines if they don't feel like they're getting the right information or they are not being heard.

narrator: gabriella makstman wants to vaccinate her two children, but she's chosen not to follow the recommended vaccine schedule. makstman: so, the plan is to be fully vaccinated as soon as possible, but we're doing one vaccine at a time. i don't know if that's the right way. i don't know where we came up with that. yeah, i don't know. narrator: yuliya patsay has a four-year-old and is expecting another child soon. she delayed vaccinating her oldest till she was three. i was concerned that her immune system couldn't handle it, and we just waited. my son and her, they're not vaccinated yet, and my older ones don't have boosters. narrator: marianna fastovsky has four children. she vaccinated at first, but then one child had a seizure. fastovsky: i'm just really worried about reactions. and i am worried about the diseases. so i'm kind of confused, really. narrator: in america, children must be vaccinated

before they start kindergarten. but the required shots vary from state to state, and most allow for exemptions based on personal or religious beliefs. here in california, almost three percent of children are exempt, and in some schools it's more than 30%. i have a lot of friends who don't vaccinate at all, and if you say "vaccine" around them, they look at you like you are literally, well, you know, like you were poisoning" your child., on the other hand, you have parents that they can't even understand why this is even a question. makstman: nobody is willing to really have a conversation with you and discuss what's a severe reaction, is it okay to have a seizure... i would really like to know what the real risks are. 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 biases. mine is that i work in a hospital, and so, for example, in 1991 in the city of philadelphia there was a massive measles epidemic. of the nine children who died during that 1991 epidemic, seven of them died in our hospital. so we had to stand by and watch while we tried to support them. and you know, as doctors your job is to try and save children's lives, and when you stand helplessly by and watch them die, it's, um... it just gets to you. narrator: human history is scarred with stories of deadly germs destroying lives. 500 years ago, about one in three children died before the age of five. but then, we began to fight back.