All states allow parents to forgo vaccines for their children medical reasons, and most offer the same right for religious reasons, but 19 states allow parents to skip the shots for philosophical reasons. California is one of those states, and had a pertussis, or whooping cough, comeback in 2010, sickening more than 9,000 people and killing 10 infants.
Marin County, which was hit hardest by the outbreak, conducted a survey two years ago to better understand what happened.
"The notion of the hard core anti-vaxxer, I think, really doesn't capture the reality," said Dr. Matt Willis, a Marin County public health officer, adding that many parents who were opting out of vaccines for their children weren't skipping all vaccines.
"It's a much more nuanced picture. Many of our parents were picking and choosing between vaccinations depending on what they thought the risk-benefit was on case-by-case basis."
He said many Marin parents got their children the pertussis vaccine after the 2010 outbreak.
About 58 percent of students at Marin County's San Geronimo Valley Elementary School were not fully vaccinated because of personal belief exemptions, according to vaccination data collected in the fall and posted on the California Department of Public Health's website.
San Geronimo principal Laura Shain said parents opt out of all recommended immunizations for various reasons, including fear of adverse reactions to the shot, fear of putting something foreign in their children's bodies and the misinformation that they no longer need to worry about viruses like measles.
But since the start of the school year, Shain said, the personal belief exemptions have fallen to 23 percent. She said the enrollment of two students with weakened immune systems has pushed parents to vaccinate to ensure that their children's classmates are safe.
"The possibility of a measles outbreak here has made it especially threatening to us and scary," Shain said. "We are not lax about [vaccinating]. It is something we encourage."media:28719142
But in Knox County Ohio, where Ella Miller lives, people in her Amish community had been vaccinating routinely until a single baby appeared to be changed by a routine shot many years ago, she said. It was enough to steer the Amish community away from immunizations, she added.
"We were really scared,” Miller, 38, told ABC News. “A lot of people just red-flagged it.”
"It spread so fast that we couldn't stop it," she said, adding that the measles spread to a larger Amish community in Holmes County, too. "A lot of people in our community probably didn't have shots. It just spread like wildfire."
She said she started feeling like she had the flu, and soon was so nauseous and dehydrated that she just wanted to pull the curtains shut and sit in the dark for a week until it was over.
"It's terrible," Miller said. "Some people ended up in hospital they were so sick. They were hyperventilating. It was awful."media:28719092
But in 1994, the Centers for Disease Control and Prevention reported a measles outbreak among Christian Scientists in the Midwest.
It started with a 14-year-old Christian Science high school student who picked up the measles virus skiing in Colorado, where a measles outbreak was ongoing, according to the CDC. The student carried the virus back to a Christian Science community college in Illinois and then to a Christian Science boarding school in Missouri. In less than a month, 191 people got it in St. Louis County, Missouri, and 49 people got it in Jersey County, Illinois, the CDC reported.
Immunization rates do not exist for this specific population, and Christian Science representatives have not responded to ABC News requests for comment. media:28718944