The Ventura County Public Health Department confirmed one case of measles that could lead to more.
The affected person is a female exchange student from Germany living with a host family in eastern Ventura County.
Symptoms include fever, runny nose, cough, red eyes and a rash.
The Centers for Disease Control and Prevention said some patients require hospitalization.
Dr. Paul Russell, Ventura County public health officer, said the case highlights the importance of all people receiving required immunizations.
Russell said the teen's parents declined the measles, mumps, and rubella vaccine that protects people from what is commonly called the German measles.
Russell said measles is so contagious that any child who is exposed to it and is not immune will probably get the disease.
Health care providers should remind patients who are traveling outside North America about the risk of measles and offer vaccine if appropriate.
VCPH’s clinics offer measles immunizations. They are recommended for all children entering school.