Members of the anti-vaccine movement said the public backlash had terrified many parents. “People are now afraid they’re going to be jailed,” said Barbara Loe Fisher, the president of the National Vaccine Information Center, a clearinghouse for resisters. “I can’t believe what I’m seeing. It’s gotten so out of hand, and it’s gotten so vicious.”

In San Geronimo, Calif., a mostly rural community of rolling hills and oak trees about 30 miles north of San Francisco, 40 percent of the students walking into Lagunitas Elementary School have not been inoculated against measles, according to the school’s figures. Twenty-five percent have not been vaccinated for polio. In all, the state says that 58 percent of Lagunitas kindergartners do not have up-to-date vaccine records.

“A lot of people here have personal beliefs that are faith based,” said John Carroll, the school superintendent, who sent a letter home to parents last week encouraging them to vaccinate their children. The faith, Mr. Carroll said, is not so much religious as it is a belief that “they raise their children in a natural, organic environment” and are suspicious of pharmaceutical companies and big business.

Some parents forgo shots altogether. Others split vaccine doses or stretch out their timeline, worried about somehow overwhelming their children’s immune system. Kelly McMenimen, a Lagunitas parent, said she “meditated on it a lot” before deciding not to vaccinate her son Tobias, 8, against even “deadly or deforming diseases.” She said she did not want “so many toxins” entering the slender body of a bright-eyed boy who loves math and geography.