Parents with students at Huntington Beach High School have been warned that a student infected with measles was recently on campus, and unvaccinated students reportedly are being kept out of school for three weeks.
In a letter sent to parents, Matt Zahn, medical director with the Orange County Health Care Agency, said the infected person was on the high school campus between Jan. 6 and 8.
The healthcare agency has been contacting staff and students who may have been exposed to measles and may not be immunized.
School officials said there would be more information released later Tuesday morning.
"Measles spreads very easily by air and by direct contact," Zahn wrote. "Simply being in the same room with someone who has measles is sufficient to become infected."
Zahn warned parents and students were at risk of contracting measles if they never had it and have not been vaccinated.
District officials told CBS Los Angeles that 24 unvaccinated students were banned from campus after being exposed to an infected classmate. They were told not to return to campus until Jan. 29.