In the latest salvo in the vaccination wars that have shaken California since a measles outbreak originated at Disneyland last month, the father of a 6-year-old boy with leukemia has asked the superintendent of his Marin County school district to keep unvaccinated children out of school.

"I respect people’s choices about what to do with their kids, but if someone’s kid gets sick and gets my kid sick, too, that’s a problem," said the father, Carl Krawitt of Corte Madera, California, whose son, Rhett, was found to have leukemia in 2010. "What we need to do, for all our children, is increase the herd immunity."

Rhett is in remission now, but four years of chemotherapy have left him vulnerable to infection and unable to be vaccinated. Until his immune system strengthens, his best protection from infectious diseases is the so-called herd immunity of a community where almost everyone has been vaccinated.

But he lives in a part of the Bay Area where an unusually high number of parents refuse vaccinations for their children. Overall, about 7 percent of the children at Rhett’s school, Reed Elementary, are unvaccinated – a rate that is higher than the statewide average but far lower than at some other schools in the county, where fully half of the students are not vaccinated, according to Dr. Matt Willis, the county health officer.

The legal authority for barring children from school rests with Willis, who said that although he was sympathetic to Krawitt’s concern – and delighted that he was raising awareness of the importance of childhood vaccines – such a measure would not be appropriate, given that there has not been a case of measles in Marin County for years.

"I obviously have to balance the responsibility to control communicable disease with everybody’s right to freedom, and the line right now would basically be if there’s a case in this school," Willis said. "This is a decision being made and applied to a school community based on the fact that these children are collected together at the school."

Steven Herzog, the superintendent of the Reed Union School District, said that Rhett Krawitt’s school placed medically fragile children in classrooms where as many students as possible were vaccinated. In Rhett’s first-grade class, he said, 19 of the 22 children have been vaccinated, excepting only Rhett and another child who is medically fragile, and one whose allergies preclude vaccines.

Krawitt, though, says that not enough has been done to ensure real herd immunity.

"It’s not just schools where diseases can spread," he said. "It’s the library, the playground, the airport, the whole community."

As vaccine resistance has spread among parents, childhood diseases that were considered eradicated have been making a comeback. Measles has spread in California and other states since December, with cases traced to an outbreak that began at Disneyland. As of Friday, there were 68 cases in California, leading officials in Orange County to bar some unvaccinated children from going to school. Related cases of measles, which had been declared eliminated in the United States in 2000, have also appeared in Utah, Washington, Oregon and Colorado, and also in Mexico.

Last year, the Centers for Disease Control and Prevention reported 644 cases of measles from 27 states year, by far the largest number since 2000. Before measles vaccines became commonplace in 1963, about 3 million to 4 million Americans a year contracted the disease, the agency said, and 400 to 500 died from it.

While a growing number of parents nationwide have opted out of vaccinating their children over the past decade, often through fear of a link between vaccines and autism, the tide has apparently been turning in Marin County, in the north San Francisco Bay Area.

According to Willis, vaccination rates in the county rose 20 percent from 2012 to 2014, as diseases like whooping cough and measles spread, convincing some parents who had been hesitant about vaccines that the biggest risk was avoiding vaccines.

And as of January 2014, California made it more difficult for parents to opt out. Previously, they could have a "personal belief exemption" just by signing a piece of paper. Now they must receive a signature from a doctor.

But Krawitt said he saw dangers to his son that these changes could not offset.

"Rhett was just getting back to being a regular kid when this measles thing came up," Krawitt said. "He still has to go back every month for blood work, and now he asks, ‘Can I get immunized yet?’"