The Center for Disease Control and Prevention is monitoring the spread of measles in 21 states ... and California is one of them.
So far this year, 107 cases have been reported. Most of the people infected were not vaccinated.
The airborne virus is spread through coughing and sneezing. Symptoms usually appear with 10 to 14 days of exposure.
CDC officials are predicting that 2018 will be one of the worst years for the illness in the past decade.
The United States had a record number of measles cases in 2014, with 667 cases spread across 27 states.
Measles is still a common occurrence around the world, including in many European, Asian and African counties. Travelers who are sick can often bring the virus to America.
Other states being monitored include: Arkansas, California, Connecticut, Florida, Illinois, Indiana, Kansas, Louisiana, Maryland, Michigan, Missouri, Nevada, New Jersey, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Tennessee, Texas, and Washington, and Washington D.C.