Public health officials in Silicon Valley said an adult who contracted measles spent time at Google’s Mountain View campus last week.
Authorities described the person as a San Mateo County resident.
Those most at risk are people who have not been vaccinated against measles, officials said. The majority of people with measles this year in the U.S. were not vaccinated, according to the U.S. Centers for Disease Control and Prevention.
If a person infected with measles walks into a room, the virus can stay there for two hours after the person leaves, ready to infect. But the measles vaccine is 97% effective against the disease.
California is one of 19 states that have reported measles cases this year.
Between Jan. 1 and April 4, 465 people nationwide were diagnosed with measles, making it the second biggest outbreak since 2000, according to the CDC. During all of last year, 372 measles cases were reported, according to the agency.
Symptoms of measles, which include a rash, can take as long as three weeks to appear.
Last month, officials warned that passengers at Los Angeles International Airport may have been exposed to measles after a passenger who had a layover in the airport was diagnosed with the disease.
Then, last week, Long Beach officials announced that a traveler diagnosed with measles passed through Long Beach Airport twice while infectious. Travelers who were at the airport March 30 or April 7 between 6 and 8 a.m. could be at risk of catching measles, officials say.