Unvaccinated students at Yuba River Charter School near Nevada City are at particular risk for contracting measles, a contagious disease that usually causes fever and other cold symptoms for two weeks but could develop into more serious ailments, including pneumonia and encephilitis.
Unvaccinated students have been told to stay home for two weeks to be sure they have not been infected and to be quarantined at home during that time, according to the San Francisco Chronicle newspaper.
Officials with the California Department of Public Health said the child developed symptoms after returning to the United States from an overseas trip.
He was last in school on March 17, the newspaper said.
Only 77 percent of Nevada County kindergarten students reported being fully vaccinated at the start of the school year, which began in September 2015.
The figure is one of the lowest vaccination rates in California.
But the Yuba River Charter School attended by the infected child reported only 43 percent of children being fully vaccinated, the newspaper said.
The low vaccination rates prompted state lawmakers to approve stricter vaccination requirements statewide.
The measles vaccine is considered 95 to 99 percent effective in preventing illness, the newspaper said.
The school is expected to reopen today.
California experienced its largest measles outbreak in 15 years last year, when about 130 people contracted the disease after coming in contact with people believed to have visited two Disney theme parks in Southern California.
But the stricter requirements, which make it more difficult for parents to refuse to have their children vaccinated, do not take effect until the 2016-17 school year, the newspaper said.
Measles symptoms — high fever, runny nose, coughing and watery, red eyes — usually begin from one to three weeks after exposure, and a distinctive rash rash develops from two to four days after the fever.
Patients are contagious several days before and after the rash appears.
Most people with measles experience mild symptoms for up to two weeks, but about 20 percent develop pneumonia and up to 2 percent develop encephalitis or die, the newspaper said.
