California is on the brink of passing a law that would require nearly all children to be vaccinated in order to attend school. The bill has cleared most major hurdles, but public health officials have grappled with a strong, vocal opposition along the way.
There's actually a long history to the anti-vaccination movement.
"From the moment the very first vaccine came on the scene, which was the smallpox vaccine, there has been resistance to vaccines and vaccination," says Elena Conis, a history professor at Emory University and author of Vaccine Nation: America's Changing Relationship with Immunization.
She says that the modern-day resistance movement shares its roots and rhetoric with the social movements of the 1960s and '70s, including feminism, environmentalism and consumer rights.
"They encouraged people to question sources of authority, including doctors," she says.
For example, women's advocates started to question medical advice on reproductive health and childbirth.
"Women also start opting to increasingly use midwives, have births outside the hospital," she says, "And also reject professional advice about formula feeding over breastfeeding."
Environmental activists were also encouraging people to think about chemical exposures, even in small amounts, at the same time that drug manufacturers started including package inserts listing drug ingredients.
Conis says that this is the context for the list of vaccine recommendations, which has grown longer over the last generation. It can be overwhelming for today's parents to watch their babies cry through one shot after another.
"We started looking at the vaccine schedule and how intense and frequent these vaccines seem to come up," says George McCann, a general contractor and father of two daughters, who lives north of San Francisco. "So we started talking about whether or not this seemed to be the best approach for our children."
He and his wife decided to have their girls get some vaccines, but not all. They skipped vaccines for pneumonia and chicken pox and waited on polio until the girls were older.
"The whole issue for me comes down to the idea that somehow the state would get to mandate that all of us have to do something, as if we don't have the ability to look into this with compassion and intelligence and critical thinking on behalf of our children," he explains.
But Carl Krawitt asks that those who don't vaccinate or delay think about other peoples' children. His son couldn't get vaccinated for five years while he was being treated for leukemia. He depended on others to be immunized so they couldn't spread potentially deadly diseases.
"People often don't understand that their choices have an impact on others," he says. "People take personal freedoms to such an extreme that they forget about the community."
These are the types of parental debates Dr. Matt Willis is navigating. He's the public health officer for Marin County. In some communities there, only half the kids are fully vaccinated.
His office is trying to figure out why. It did a survey and found a few common characteristics of today's parents who don't vaccinate.
"A higher proportion are getting information from the internet, and a higher number of the parents were seeing alternative medical providers," he says.
Willis has developed a list of talking points for each vaccine. He tells parents that there's no link between the measles vaccine and autism.
He says that polio is probably his toughest sell. The disease was eradicated from the United States in the late 1970s, so American parents today have no memory of how horrible the disease was. While the polio virus is not endemic to the U.S., he reminds parents that it still is in other parts of the world.
"It's really just one plane ride away," he says.
Willis tell parents who want to delay some vaccines to think of them like a seat belt.
"You could choose to put them in their safety belt as you leave your driveway and start driving, or you could choose to pull over 10 miles later and put it on," he says.
Willis is hoping California Gov. Jerry Brown will sign the bill prohibiting parents from opting out of vaccines for religious or personal beliefs. If passed, the law would take effect on Jan. 1, 2016.
"It will certainly make my job a lot easier," he says, because controlling a disease outbreak is like fighting a fire. "It's much easier to prevent a fire from happening in the first place than it is to try and extinguish it once it's spreading."
This story is part of a reporting partnership with NPR, KQED and Kaiser Health News.