A vial of the measles, mumps and rubella vaccination for children, shown in Los Angeles on Oct. 24, 2025. 

Public health officials in Napa County confirmed a case of measles on Wednesday. 

It’s the third measles case detected in the Bay Area since December. In addition, health officials in Santa Clara County confirmed on Wednesday that a sample of wastewater tested positive for the measles virus.

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The case in Napa County occurred in an unvaccinated child, who became ill after traveling to South Carolina, where a large outbreak of the highly contagious virus is currently underway. 

“This is exactly how outbreaks start,” Matt Willis, a former public health officer for Marin County, told SFGATE via email. “Every outbreak begins with one case, usually someone who’s infected outside of the community.” 

The case marks the county’s first reported measles infection since 2012. All three recent cases were identified in unvaccinated individuals; one was reported in San Mateo County, while the other was reported in Contra Costa County.

“This child was infected outside of the state,” Willis noted of the Napa case. “This is a concerning sign of the local impact of the national failure to maintain adequate vaccine coverage.”

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