 -- California public health officials are working to stop a possible measles outbreak after a student tested positive for measles at a school with a high number of unvaccinated students.

The Nevada County Public Health Department said the unnamed student attended the Yuba River Charter School and showed symptoms when the student attended school earlier this month. The agency did not identify the grade of the student. Nevada county is in the Sierra Nevada mountains, about an hour north of Sacramento and near the border with Nevada.

Dr. Karen Smith, the director and State Public Health Officer at the California Department of Public Health, said the child showed measles symptoms after traveling overseas. The child has recovered from the illness, but health officials said they are concerned that other unvaccinated children may have been exposed.

“As the state’s public health officer, it’s concerning to receive a report of a child with measles because it’s a disease that can easily be prevented,” Smith said. “Immunization is the best way to protect against measles. Two doses of the Measles, Mumps, and Rubella vaccine are approximately 97 percent effective at preventing disease in exposed persons.”

Measles is one of the most infectious viruses on the planet. Unprotected people exposed to the virus have a 90 percent chance of being infected. The virus is airborne and an unprotected person can be infected if they simply enter the same room an infected person was in hours earlier.

The Yuba Charter School has a low rate of vaccine compliance and is classified as "most vulnerable" to outbreaks, according to data from the California Department of Public Health. The overall vaccination rate for the school's kindergartners is just 42.6 percent, according to the California Department of Public Health. Only staff and students who are up to date on their vaccines are currently allowed to come to school, according to the school's website.

The California Department of Health collects data on vaccination rates for three cohorts -- day care, kindergarten and 7th grade.

About 44 percent of kindergartners at the school are up-to-date on their MMR (measles, mumps and rubella) shots, according to the California Department of Public Health. To create an effective "herd immunity" that can help stop outbreaks, health officials advise having at least 95 percent of people vaccinated.

Symptoms of measles include diarrhea, ear infection and pneumonia. Rare severe complications occur more often among children under five, those with compromised immune systems, and adults over the age of 20 are at higher risk for severe complications such as encephalitis or death.