Belvedere Island in Marin County, California, is one of the highest-income cities in the state. Marin County has become the epicenter of a the country’s anti-vaccination movement, a trend that goes back at least a decade and has been largely to blame for California’s ongoing measles outbreak. Its followers are generally affluent, liberal and well-educated. Creative Commons

Marin County, California, an enclave of expensive homes, private schools and yoga studios nestled along the Pacific coast just north of San Francisco, has been considered by many to be the epitome of active, healthy living. But in recent years the birthplace of modern mountain biking and competitive trail running has become the epicenter of the country’s anti-vaccination movement, a trend that goes back at least a decade and has been largely to blame for California’s ongoing measles outbreak that has put health officials and parents on edge.

Its followers are generally affluent, eco-friendly, liberal and white – parents with college educations whose families travel abroad, eat well, drive Priuses and attend private schools. “You’re talking about a place where they only eat organic everything,” said Lexie Jordan, a 25-year-old yoga instructor in New York City who grew up in Marin County. “People spend money on these things, like all the moms go to yoga, and everyone’s buying organic from Whole Foods or these huge farmers markets. ... I’m sure people don’t even use antibacterial stuff there."

Despite its reputation for being physically fit and contaminant-free, Marin County has a serious public health problem. Too few parents have chosen to vaccinate their children against such contagious diseases as measles and pertussis, commonly known as whooping cough. So-called anti-vaxxers have often eschewed modern medicine for more traditional health regimens, turning to the Internet’s flood of health and wellness blogs for direction. Blog posts with titles like “Massive Vaccine Cover-up Confirmed: Secret Documents Prove Vaccines Cause Autism” and “Studies Show That Vaccinated Individuals Spread Disease” inflate the risks of childhood vaccinations but end up getting disseminated throughout Marin.

In many ways, the very ideologies and habits that have made Marin County among the healthiest counties in the state are the same that gave rise to more parents not vaccinating their kids. "There is a fear or wariness here of things that don’t occur naturally," said Michelle Funez, a 44-year-old Marin County fitness blogger who runs a cognitive behavior psychotherapy practice for weight loss and maintenance. "It makes sense that something like a vaccination that was made in a laboratory, someone would see that as fiddling with nature." People generally live by the adage "If it’s natural, it’s probably better," said Funez.

Many anti-vaxxers said they believed vaccines were toxic and latched on to wild health claims such as the link between vaccines and autism, an idea that began circulating in the late 1990s and has since been debunked. Other anti-vaxxers said they thought that administering too many at once could somehow overload the child’s immune system.

“The communities that are vaccine-hesitant tend to be also more hesitant overall toward mainstream health care systems,” said Matt Willis, Marin County’s public health officer. “There’s certainly an adherence to a philosophy of purity, that some people see vaccines as being a kind of threat too.” Parents believed they were doing what’s best for their kids, health officials have said, with some parents even thinking that by allowing their children to be exposed to the virus, they’re building up their immunity naturally.

Marin children were nearly twice as likely in recent years not to have received the measles, mumps and rubella, or MMR, vaccine, compared with the average California kindergartner, according to county officials. Full vaccine opt-out rates peaked in Marin in 2012 at 7.8 percent – more than twice the state’s average. Underimmunization rates, in which children received some but not all federally recommended vaccines, were as high as 17.9 percent. Nonvaccination tends to be higher at Marin County’s private schools compared with its public schools. Some classrooms had nonvaccination rates as high as 74 percent, according to officials. With the state’s measles outbreak on most every California parent’s mind, health officials in Marin have feared that just a few cases could spark an outbreak.

In fact, the fuse already may have been lit. Marin County recently reported its first two cases of measles in 14 years – two siblings whose infections were linked to the Disneyland outbreak in Southern California that began in December and has spread to 14 states and Mexico.

Some health practitioners in Marin have pushed back against the anti-vaxxers by denying services to certain unvaccinated groups. In 2012, Tamalpais Pediatrics in the town of Greenbrae, in the heart of Marin County, told its patients it would no longer accept unvaccinated kids over the age of 2 into its clinics. “We live in an affluent community, a lot of people travel,” said Ira Bagshaw, practice manager at Tamalpais Pediatrics. “We were looking at the Summer Olympics in London that year, a lot of our patients’ families were going. ... There were a lot of cases of measles [in the U.K.] at the time.”

Marin County’s median household income in 2013 was $91,000, 33 percent higher than California overall, according to the U.S. Census Bureau. The average Marin County home that year was valued at about $782,000 – more than twice the value of the average California home. Its population was more than 86 percent white.

The majority of Marin County residents, however, are not anti-vaccine. “Most of the discussion that I hear about it are people who disapprove of the anti-vaccine families,” Pamela Fox of Marin Mommies said over email. Public health officials have described the anti-vaxxers as being part of a minority.

Generally, vaccination rates in the U.S. have remained high. One thing anti-vaxxers have had on their side has been “personal belief exemptions.” While all states have required all children entering kindergarten to get vaccinated for measles, mumps and rubella, among other infectious diseases, 20 states have allowed parents to forgo immunizing their kids based on personal or religious beliefs, according to the U.S. Centers for Disease Control and Prevention. All states allow medical exemptions for things like allergies or cancer.

In 2014, a record number of California parents claimed personal belief exemptions for their children. Not coincidentally, the state also recorded its most measles cases in 20 years and the highest rate of whooping cough cases in 68 years, according to California health officials.

The U.S. officially declared measles a disease of the past, like smallpox or the bubonic plague, in 2000. Despite vaccines’ efficacy and proven safety, leaders of the anti-vaccination movement have insisted that parents should be able to choose to do what they want with their children. “They’re being pressured to have their kids vaccinated,” said Barbara Loe Fisher, co-founder and president of the National Vaccine Information Center, a controversial group that has often been cited as anti-vaccine. “Doctors should be partners with patients and parents, not dictating and compelling people against their will.”

Choice might soon be off the table for many Californians. Earlier this week, state legislators proposed eliminating personal belief exemptions in the state. The law would prohibit parents from citing religion or personal choice as reasons not to vaccinate their kids before enrolling them in public or even private schools.

The major reason for the rise in anti-vaxxers, public health officials have said, is that the disease simply hasn’t been as visible as it once was. “If you say to me ‘head lice’ or a variety of other childhood diseases, I can go to a file, open it up and see the letter that we send to parents,” said Mary Jane Burke, Marin County Superintendent of Schools. “Measles, no, because no one gets measles anymore.”

Health officials in Marin have become concerned that the parents who have not vaccinated their children against measles are leaning too heavily on pseudo-science and personal convictions over the greater good of the community. “If in the end of the story, the spotlight on Marin provides insight to the entire state and to the country about the importance of immunization, I’m very supportive of that," Burke said.