ARCHIVAL (NBC, WORLD NEWS TONIGHT, 4-24-19):NEWS REPORT: Measles, the most contagious disease in the world spreading in hot spots around the country.

NARRATION: In 2019, the United States hit an alarming new record.

ARCHIVAL (CBS, EVENING NEWS, 4-24-19):JEFF GLOR: Measles surges to a 25-year high.

NARRATION: Its the latest in a series of outbreaks that are drawing attention to a growing problem.

ARCHIVAL (NBC, 8-27-13):DR. LAURA POPPER: Some people are deciding theyre afraid of vaccines.

NARRATION: The return of measles isnt just a danger to those who refuse to vaccinate. Newborns who cant get inoculated and other vulnerable people depend on something called herd immunity to protect them. Take whooping cough. It requires one of the highest percentages of the population to be immunized to prevent the disease from spreading. If not, the consequences can be dire.

ARCHIVAL (CBS EVENING NEWS, 9-23-10):KATIE COURIC: The risk of whooping cough may sound like something from the past but its still very real. Today California reported more than 4,200 cases. Nine people have died all of them infants.

NARRATION: For San Francisco mother, Mariah Bianchi, those numbers are more than just statistics. When her son was born in August 2005, as a nurse, she realized something was wrong.

MARIAH BIANCHI: It was just like he was so lethargic. And I knew that there was just something. Im like, I cant keep him awake. We went to the doctor and she said, I want you to go to the hospital. As soon as he got there, he went into cardiac arrest.

NARRATION: What she didnt realize was that the immunity from her own whooping cough vaccine had worn off and her newborn was too young to be inoculated.

MARIAH BIANCHI: And they started CPR right away, for probably about 45 minutes or so. As a nurse Im thinking, I know what that means. Your brain is not getting oxygen. Your body is failing. And the surgeon came out, and he said, his chance of survival is very low. We made the most compassionate decision you could, but we just said, you know, dont-dont do it. Just stop.

NARRATION: Dylan Bianchi died 17 days after he was born.

So how did society get to the point where some populations are left vulnerable?

Vaccines are one of the greatest achievements in public health.

ARCHIVAL (MYFOOTAGE.COM):ANNOUNCER: Dr. Jonas Salk discovers a vaccine that promises to wipe out childhoods crippling and killing enemy polio!

NARRATION: Polio, smallpox, diphtheria no longer a threat in the US because of vaccines. And in 2000, another watershed moment.

ARCHIVAL (NBC, NIGHTLY NEWS, 9-2-99):JANE PAULEY: The CDC reports the measles, practically wiped out tonight, in the United States.

NARRATION: But that report proved to be overly optimistic.

SETH MNOOKIN (MASSACHUSETTS INSTITUTE OF TECHNOLOGY): The measles vaccine has been so effective it doesnt seem like something we need to protect our children from. You have this, sort of, fundamental paradox of vaccines, that theyve become a victim of their own success.

NARRATION: Science writer Seth Mnookin examines the fear about the measles vaccine in his book, The Panic Virus. He says it can be traced to a moment in the late 1990s.

SETH MNOOKIN: The current vaccine scares and controversies that were still dealing with today stem from a 1998 paper that appeared in The Lancet, a very respected medical journal published out of the U.K.

NARRATION: The paper, written by Dr. Andrew Wakefield, claimed there might be a connection between the measles, mumps, rubella vaccine and autism.

SETH MNOOKIN: In his press conference, Andrew Wakefield stood up and said, Parents should not give their children the MMR vaccine, period, until we are able to get to the bottom of this.

ARCHIVAL (ITN, 1998):ANDREW WAKEFIELD: The MMR vaccination in combination that I think it should be suspended in favor of the single vaccines.

SETH MNOOKIN: And what the media in the U.K. did was, they ran with that.

ARCHIVAL (ITN):REPORTER: Doctors at the Royal Free Hospital believe they may have discovered a link between the combination vaccine and a bowel disease that can progress to autism.

SETH MNOOKIN: The notion that you would take a 12-person case study and make claims about a population as a whole is ridiculous. This paper was historically bad.

NARRATION: It was later revealed that Wakefield also had a financial stake in trying to establish a connection between the MMR vaccine and autism. Wakefield denies the allegations, but records show he was paid more than 435 thousand pounds.

SETH MNOOKIN: Andrew Wakefield was receiving money from a lawyer who was working with parents intent on suing vaccine manufacturers. Perhaps the most shocking revelation is that he faked some of the data. Hes lost his medical license, The Lancet paper has been retracted, but he had very effectively positioned himself as a martyr and in some odd way, every piece of evidence that comes out against Wakefield, sort of, solidifies his standing in the community that still pays attention to him.

NARRATION: Follow up studies of hundreds of thousands of children did not find any evidence that the MMR vaccine causes autism.

But early on, fears about vaccine safety took hold because complicated science proved difficult for public health institutions to communicate. Case in point, their response when concerns were raised over a vaccine preservative called thimerosal, which contains ethyl mercury.

ARCHIVAL (C-SPAN):FORMER REP. DAN BURTON (R-INDIANA): Children are getting mercury injected into their bodies with vaccines!

ARCHIVAL (CBS, 7-15-05):SHARYL ATKISSON: Thats right, mercury, a known neurotoxin.

NARRATION: But ethyl mercury in thimerosal is not the same as the toxic methyl mercury, which is found in fish and accumulates in the body. Nevertheless, the Public Health Service and the American Academy of Pediatrics recommended thimerosal be removed. And their messaging backfired.

ARCHIVAL (CBS, 5-18-04):SHARYL ATKISSON: In 1999, health officials denied a link between vaccines and the autism epidemic, yet urged vaccine makers to take out the mercury just to be safe.

SETH MNOOKIN: What the American Academy of Pediatrics said, is, We are recommending this step so we can make safe vaccines even safer. As a parent, if you tell me somethings safe, I dont think thats on a sliding scale. I assume that, if you say its safe, it is safe for my child. Its not safe, safer, safest. There are almost two languages here. Theres the language of science, and then theres English. And, in the language of science, you have these signifiers like, to the best of our knowledge, as far as we know.

ARCHIVAL (CBS, EVENING NEWS, 5-28-04):DR. STEVE COCHI (CDC IMMUNIZATION PROGRAM): Based on the available scientific evidence

SETH MNOOKIN: Because you cant say anything with 100%. You cant prove a negative. And so, when scientists speak in their language, and the rest of us translate that into English, it sounds like theyre saying something very different than theyre saying.

ARCHIVAL (CBS, EARLY SHOW, 7-15-05):DR. TANJA POPOVIC (CENTERS FOR DISEASE CONTROL): Based on what we know right now, we dont think that there is an association.SHARYL ATTKISSON: But thats not saying with 100 percent certainty there isnt one.DR. TANJA POPOVIC: That is saying, that based on the evidence we have right now, we dont think there is one.

SETH MNOOKIN: Either because the reporter doesnt understand whats actually going on, or because theyre looking to generate a story, they then take that and make it seem as if the scientist is saying, I think theres a possibility that vaccines do cause autism when, in fact, thats not it, at all.

BRENDAN NYHAN: News organizations should exercise judgment about what goes out over their air.

NARRATION: Brendan Nyhan is a professor at the University of Michigan who studies how misinformation spreads, and the role of the media.

BRENDAN NYHAN (UNIVERSITY OF MICHIGAN): Whats particularly important is to think about the overall scientific consensus. Where is the weight of the evidence? And is our reporting reflecting that or not? Thats whats often gone astray in the vaccine debate.

ARCHIVAL (NBC, THE TODAY SHOW, 10-30-08):DR. NANCY SNYDERMAN: Its time for everyone to redirect the questions toward finding the cause of autism. It is not however vaccinations.MATT LAUER: Controversial subject, Nancy.NANCY SNYDERMAN: Not controversial subject!MATT LAUER: Well, controversial for parents who still believe.NANCY SNYDERMAN: It is not controversial, Matt, it is time for kids to get their vaccines.

BRENDAN NYHAN: Everyday people cant be fact checkers for every story about vaccines. When journalists dont give people the weight of the scientific evidence, theyre letting them down.

ARCHIVAL (CBS, 5-18-04):JENNIFER LASSITER (MOTHER): She got her vaccinations, she ran a low grade fever, she got a little rash, and then she stopped talking.

NARRATION: A false sense of balance was also created when scientific evidence was equated with peoples personal experiences.

SETH MNOOKIN: Reporting fell into this, on the one hand, on the other hand fallacy, this notion that if you have two sides that are disagreeing, that means that you should present both of them with equal weight.

ARCHIVAL (ABC, OPRAH, 9-24-07):JENNY MCCARTHY: We vaccinated our baby and something happened. Thats it.

SETH MNOOKIN: Jenny McCarthy has had more to do with popularizing the notion that vaccines are dangerous than any other single person in the United States.

ARCHIVAL (CNN, LARRY KING LIVE, 9-27-07):LARRY KING: We begin of course with Jenny McCarthy, the actress, and entertainment personality. Her son Evan has autism.

SETH MNOOKIN: Shes very smart, shes telegenic.

ARCHIVAL (CNN, LARRY KING LIVE, 9-27-07):JENNY MCCARTHY: Look at, its plain and simple. Its bullshit!DOCTOR: No, its not.JENNY MCCARTHY: Yes, it is.DOCTOR: Excuse me.

SETH MNOOKIN: When I look at clips of her, its a completely unfair fight.

ARCHIVAL (ABC, OPRAH, 9-24-07):JENNY MCCARTHY: My science is named Evan. Hes at home. Thats my science.

SETH MNOOKIN: Jenny McCarthy has said many times, and oftentimes very loudly, that her child is her scientific fact. Any scientist or any science reporter whos familiar with how science works would say that, No, any one person is an anecdote, and the plural of anecdote is not data. You know? Its just a story.

NARRATION: But stories are powerful. While vaccination rates are high nationwide, there are some religious enclaves and communities of well-educated, upper middle class people where vaccine hesitancy runs strong.

SETH MNOOKIN: I was interviewing an epidemiologist and he said, Oh yeah, we completely know where were going to have communities that have issues with vaccine uptake. We take a map and stick a pin wherever theres a Whole Foods and draw a circle around it, thats where were going to have problems. He was obviously being facetious.

NARRATION: Exasperated health officials are trying to come up with new ways to communicate with the public. Brendan Nyhan conducted a study and watched how hesitant parents reacted when they were shown information from the CDC website stating theres no evidence the MMR vaccine causes autism.

BRENDAN NYHAN: The good news was, it did cause parents to be less likely to believe in the myth that the MMR vaccine causes autism. The bad news is, however, that it made them less likely to say they would vaccinate a child. Which is precisely the opposite of what we would hope to see. What we found is that telling people the correct information wasnt actually effective.

NARRATION: That highlights how susceptible people can be to misleading information. A recent investigation found that Internet trolls from Russia targeted Americans and used the topic of vaccines to create division.

Some states are weighing inand taking action.

ARCHIVAL (CBSN):VLAD DUTHIER: California now has one of the strictest school vaccination laws in the country.REPORTER: There will no longer be religious exemptions. There will no longer be personal belief exemptions.

NARRATION: After an outbreak linked to Disneyland in 2014, California enacted one of the toughest vaccine laws in the country. Still, some parents are trying to circumvent the law.

For Mariah Bianchi, the notion of leaving some people vulnerable is hard to understand.

MARIAH BIANCHI: What does it take? How many times do you have to tell people or talk about it? We all have a role in helping each other to protect each other. A vaccine preventable disease should not have killed my son.