Christina Hildebrand went down a rabbit hole and emerged at the statehouse in Sacramento.

That’s how she describes it–going down a rabbit hole–and in her case it happened 14 years ago, when she was pregnant with her first child. In a world filled with chemicals and toxins, processed foods and GMOs, she decided her baby would be brought up as naturally and chemical-free as possible. It was when she was researching how best to achieve that goal that she bumped into vaccines.

That was a bad time to begin thinking about such things. The fraudulent 1998 paper by British physician Andrew Wakefield ostensibly linking vaccines to autism had not yet been retracted, and American celebrities, led by former model Jenny McCarthy, would soon begin making talk-show hay on that phony idea. Hildebrand didn’t like what she was hearing.

“The vaccination issue is a choice,” she says. “If you choose to be vaccinated for the measles, then you’re covered. You don’t need to worry about somebody who is not vaccinated.”

Hildebrand will not disclose what choice she made for her own children. “Their medical history is private and not something I care to share,” she said in an email to TIME. But she’s less reticent about her views on vaccines as a whole. Owner of a market-research firm in San Francisco, she is also the founder of A Voice for Choice, an advocacy group that challenges vaccine science and lobbies against state legislation that mandates vaccination as a condition for attending public schools and in favor of legislation that allows parents to opt out. The group has three part-time employees plus “a huge number of volunteers,” Hildebrand says.

She was in Sacramento on May 21 as the legislature was taking up SB 276, a bill to close a loophole that allows some parents to sidestep vaccine requirements by finding a provider willing to attest that their child cannot tolerate the shots for medical reasons. That same morning, an event for Sacramento’s Summer Food Service Program was taking place in front of the statehouse, and children and teachers in blue T-shirts were everywhere. At the edge of the crowd was a scattering of yellow–a small group of anti-vaccine mothers in yellow vests, an apparent nod to France’s gilets jaunes, the populist economic movement.

The mothers were mostly observing, there to protest a speech by California state senator Richard Pan, a physician and the sponsor of SB 276, and all but one declined to talk to TIME on the record. Representatives of anti-vaccine groups in other states, including Maine, Michigan, Vermont and Arizona, also declined to be interviewed for this story by email or phone. The California mothers were gracious, heartfelt and genuinely worried about what they believe to be the manifest danger of vaccination. But in a state as increasingly vaccine-friendly as California, they fear mockery and ostracism if they were to be marked with the anti-vax brand. (Like most such activists, they prefer to frame the issue in terms of choice and medical freedom, insisting they are not against vaccines but rather believe the decision whether to vaccinate is best left to parents. For the sake of clarity, however, the term anti-vaccine will be used for those who oppose mandatory vaccinations and pro-vaccine for those who support them.)

One of the women, Denise Aguilar, was not so wary. “We are just parents who are getting a little bit fed up with the bills that are being passed,” she said. “To have our health in bureaucrats’ hands, that’s the complete tyranny of the government.”

Activists line the halls in Sacramento during a hearing on SB 276. Rich Pedroncelli—AP

Sacramento wasn’t the only place people sharing these beliefs gathered that week. Three days earlier, hundreds protested in front of the Arizona statehouse in Phoenix, where legislation is being considered that would eliminate the state’s personal-belief exemption, which allows parents to opt out of vaccines on philosophical grounds. The scene looked festive, but sentiments were dark, with particular venom directed at the U.S. Centers for Disease Control and Prevention, which champions childhood vaccination. “Convict CDC,” read one sign with red-painted bloody handprints. “No to forced medical procedures,” read another. Children and babies wore T-shirts that made them living posters. “I get my antibodies from breast milk,” read one. (Yes, but not enough to prevent disease.) Present at the event was medical gadfly Robert F. Kennedy Jr., who has been campaigning against vaccines on the basis of false science since 2005.

Four days before the Sacramento event, a rally was held in Albany, N.Y., as the state reels from the current epidemic of vaccine-preventable measles, with nearly 600 cases in New York City and more than 260 outside the city. Kennedy showed up in Albany too.

In Austin, where the state legislature took up bills both loosening and tightening vaccine rules before its May adjournment, the group Texans for Vaccine Choice (TFVC) is a regular presence. “They are all over the place,” says Jinny Suh, founder of the pro-vaccine group Immunize Texas. “You see them at hearings where they testify about every kind of myth.”

Those states are not remotely the whole story. There are registered anti-vaccine PACs in at least four states: Oregon, Michigan, Oklahoma and Texas. Every state in the country except Alaska and West Virginia has at least one anti-vax organization, according to the Vaccine Liberation website, which tracks them. The most aggressive of the groups are not just demonstrating but also actively challenging pro-vax legislators, running candidates against them in primaries.

“I talk to colleagues, and they say that their problem is that people who support vaccines are not single-issue voters, while anti-vax people are,” says Pan, the sponsor of California SB 276. “They tell me, ‘I don’t need this faction working against me.'”

The ubiquity of the groups, of course, does not change the misinformation they spread: that vaccines are dangerous, even deadly; that they are linked not only to autism but also to ADHD, asthma, depression and a grab bag of other conditions. The near unanimous global consensus on the safety and lifesaving power of vaccines, they say, is a conspiracy driven by profits, on the part of governments, the pharmaceutical industry and even individual pediatricians.

It’s not true–any of it–and the nonsense comes at a very bad time. The World Health Organization recently labeled vaccine hesitancy one of 2019’s leading threats to global health, and in the U.S. that threat is escalating. A national measles outbreak has exceeded 1,000 cases in 28 states in just the first five months of 2019, making it the worst year for the disease since 1992. Measles had been declared eliminated in the U.S. in 2000–meaning the few infections that did turn up originated elsewhere–but if the current outbreak continues through the fall, the U.S. could lose its elimination status.

While some anti-vaxxers claim measles is merely a fever and a rash, a rite of immunological passage that confers natural lifetime immunity, public-health officials are clear that it is not. Worldwide, the disease killed 110,000 people, most of them children under 5, in 2017. In the U.S., before the introduction of the first measles vaccine in 1963, there were up to 4 million cases annually, with 48,000 hospitalizations and up to 500 deaths per year.

Measles isn’t the only resurging vaccine-preventable disease. There have been outbreaks of mumps around the country, with 1,002 cases so far in 2019–some due to college students missing their booster shots, others due to people who never got vaccinated in the first place. Whooping cough, too, has resurfaced. Part of that is due to a mutated bacterium, but earlier outbreaks–like one in California in 2014–have been tied to overuse of personal-belief exemptions.

Social media is a force multiplier in spreading the junk science, with Facebook serving as a no-cost organizing platform for many of the local groups. A 2018 study out of George Washington University found that Russian bots have been spreading vaccine misinformation across Twitter, in what experts believe to be one more attempt at sowing discord in the U.S. Even Pinterest has had to combat the propaganda, blocking anti-vax searches and pulling down offending posts.

What galls public-health experts is that the anti-vaxxers’ “personal choice” really isn’t all that personal. Depending on the disease, it’s necessary for about 95% of a population to be vaccinated to provide so-called herd immunity, the ability of a well-inoculated community to protect the few of its members who can’t be vaccinated due to age, illness or a weakened immune system. But in 2019, as anti-vaxxers put the individual over the group, the group is in growing danger.

“Vaccines are a victim of their own success,” says Dr. Paul Offit, director of the Vaccine Education Center at the Children’s Hospital of Philadelphia. “People have forgotten how sick measles can make you and how dead measles can make you.”

Ethan Lindenberger, 18, testifies before a Senate committee about getting ­vaccinated against his parents’ wishes on March 5. Carolyn Kaster—AP

The current surge in anti-vax energy is traced by most observers to Disneyland. In 2014, an outbreak of measles originating at the California theme park spread across seven states and into Canada. The next year, state legislatures began examining whether exemptions to mandatory vaccinations were too easy to come by. And the battle was joined.

Most states require children to be up to date on vaccines to attend public school–and, in some cases, private school–but parents can claim exemptions, depending on the state: religious; philosophical or personal belief; and medical–as, say, when a child is immune-suppressed or allergic to some vaccine ingredients. Only two states–West Virginia and Mississippi–had no religious or philosophical exemptions at the time of the 2014 outbreak and, no coincidence, they had the best vaccination rates in the country, with Mississippi fully vaccinating 99% of its childhood population. Nineteen other states had philosophical exemptions, and 48 also had religious exemptions.

In many states, overuse of exemptions was pushing vaccination rates below the herd-immunity threshold. Bills were introduced around the country eliminating one or both of the nonmedical exemptions, and the activists rose up to fight them. California’s SB 277, passed in 2015, was the first to go into effect, retaining only the medical exemption, and it took until this May before a fourth state, Maine, followed suit.

Exemptions aren’t the only flash points the bills address. One proposal, a favorite of anti-vaxxers, would require doctors to make vaccine package inserts available to families, in the expectation, pro-vaxxers say, that the list of chemical ingredients and possible side effects–included on all inserts for all drugs–would scare parents into saying no. Another, popular among the pro-vax community, would require doctors to enter patient vaccination records in a statewide tracking system.

“There is no bill that says, ‘We’re going to come take your children if you don’t vaccinate,'” says California assemblywoman Lorena Gonzalez, who is the co-author of SB 276, leading the fight for its passage on the assembly side. “What we say is, ‘Then you can’t take them to school because now your personal decision is going to affect other kids in the community.'”

So far this year alone, according to the website of the National Vaccine Information Center (NVIC)–an anti-vax group that is seen as a good source of information on vaccine legislation–187 bills were being considered in legislatures. Just 70 of those are anti-vaccine, but at least some of the others are unlikely to have been filed at all but for the anti-vax activity.

Even when pro-vax bills do get signed into law, there is no guarantee the anti-vaccine activists will go along. “It doesn’t matter what laws are passed,” says Aguilar, the mom who spoke to TIME in Sacramento. “We’re not going to comply with them.”

That noncompliance can happen with the connivance of doctors. No sooner had the 2015 California law gone into effect than anti-vax parents began looking for physicians who would sign a medical waiver even if they had never before met the child. One doctor alone was responsible for about a third of the 500 medical exemptions issued in a San Diego school district, according to health-department records. There was nothing technically illegal about what she was doing.

In response, Pan and Gonzalez introduced SB 276, which would require all medical waivers to be sent to and approved by the state board of health before an unvaccinated child could attend public school. The bill, which is pending in the assembly, passed the state senate on May 22–though not before hundreds of parents lined the statehouse hallways, calling the bill a “crime against humanity” and Pan himself a “tyrant.”

Warm bodies and loud voices may be the biggest advantage the activists have, because in other respects they’re outgunned, especially financially. Some, like Texans for Vaccine Choice, are 501(c)(4) nonprofits, but their funding is so low–less than $50,000 a year–that they are not required to file a long-form disclosure with the IRS. The group donated to dozens of candidates in the 2018 cycle, but its biggest contribution was $5,500; its smallest was $25. One of the best funded, according to the nonprofit investigative newsroom ProPublica, is the NVIC, whose most recently available fundraising totals–from 2015 to 2016–were just over $1 million.

That’s real money, but it doesn’t compare with that of the pharmaceutical industry, which donated more than $2 million to the campaigns of California state legislators in 2013 and 2014. Pan topped the list at $95,150, according to a 2015 story in the Sacramento Bee that is cited by opponents as evidence of bias. Pan pushes back, pointing out the industry donated to 19 other lawmakers as well, some of them his opponents. “As a pediatrician I spent an enormous amount of time studying and practicing,” he says. “I was there during the 1991 measles outbreak in Philadelphia and saw children get sick and die. If people think I need more incentive to believe in vaccines, they’re wrong.”

Such testimony from the field is unlikely to work on the most committed anti-vaxxers, who are convinced (despite all evidence) that the shots are worse than the disease. “The vaccine issue is kind of sui generis, in that the belief is so strong and unamenable to scientific evidence and the wisdom of common experience,” says Michelle Mello, professor of health law and policy at Stanford University.

It’s possible the culture will age out of anti-vax misinformation. Scattered reports of unvaccinated kids getting their shots as soon as they turn 18 could be a bellwether of healthy generational shift. In March, Ohio high schooler Ethan Lindenberger made news when he did just that, then testified about his decision to a Senate committee. In the meantime, the battles may simply have to be fought in the legislatures and at the ballot box–and for now, the science is winning. In recent elections, a number of anti-vax challengers either won primaries but lost in the general or did not even make it to the final round. Several incumbents backed by Texans for Vaccine Choice did win re-election, though their incumbency made them much surer bets.

In terms of legislation, anti-vax success has also been minimal. A 2018 study in the American Journal of Public Health reported that of 175 vaccine-related bills studied from 2011 to 2017, only 13 were signed into law–and 12 of those actually limited access to vaccine exemptions. The TFVC celebrated what it could after the Texas legislature adjourned, announcing, “NO BAD BILLS were passed!” on its Facebook page. On the TFVC blog, the group’s founder, Jackie Schlegel, took a sharper tone, swiping at this year’s “manic-media-measles-meltdown.” (As of press time, Schlegel had declined to be interviewed by TIME.)

But state legislatures meet every year, and Peter Hotez, professor and dean of the National School of Tropical Medicine at the Baylor College of Medicine, worries that the anti-vax community is adept at playing the long game. “They work on this year after year, so they’re chipping away,” he says.

Hotez believes the front lines of the fight involve social media. In early March, Facebook pledged to prevent the spread of vaccine misinformation, but it has declined to shut down anti-vax pages directly–which, as a private company, it could do without running afoul of the First Amendment. It has also promised to serve up more factual information when users search for vaccines, but both Facebook and Instagram, which it owns, continue to be rife with misinformation.

Such efforts would likely find a very receptive audience. A 2017 Pew Research Center study showed that 82% of Americans believe “healthy children should be required to be vaccinated to attend school because of potential health risk to others.” A Reuters/Ipsos poll in May found 77% of respondents believe children should be vaccinated against measles even if parents object.

None of that is likely to dissuade Hildebrand, Kennedy or other opponents of mandatory vaccines, for whom the argument comes down to the binary business of parental right to choose. In that lies the particular power–and the particular peril–they represent. Climate-change deniers may make it incrementally harder to enact smart environmental legislation. Moon-landing conspiracists may diminish the quality of debate in the public square. But every parent who chooses not to vaccinate has at least one child whose own health is being left at risk and who represents a danger to others too. Neither the child nor the community had a voice in that choice.

This appears in the June 24, 2019 issue of TIME.

Write to Jeffrey Kluger at jeffrey.kluger@time.com.