Public health officials in Alameda and Santa Clara counties have put out an alert about possible measles exposure that could have occurred at several retail locations in the last nine days.

A South Bay resident who had been traveling, returning to the US from international travel and passing through an airport where there was some measles exposure, tested positive for measles in recent days. The person is reportedly recovering at home and did not require hospitalization, but before knowing that they were infected, did some shopping last week where they may have exposed members of the public to the disease.

Health officials warn that if you were at the H Mart on Oakland Road in North San Jose on May 21 between 7:15 and 9 pm, you may have been exposed. This warning is most acute for those who are not immune or are not sure whether they are vaccinated against measles.

Also, if you were at the Starbucks on 34050 Newark Boulevard in Newark on May 23 between 11:45 am and 2 pm, or if you shopped at Trader Joe's in Milpitas on May 25 between 4:45 pm and 8 pm, you may have been exposed.

"At this point we don't know the number of people who may have been exposed to this very contagious disease," says acting Santa Clara County health officer Dr. Sarah Rudman, speaking at a news conference Friday, per KPIX.

Measles is an airborne pathogen that can remain airborne for up to two hours after an infected person leaves an area — which is why the time windows above are so extended. And an infected person can be infectious for up to four days before the onset of the signature rash, and for up to four days after.

This is the first recorded case of measles in Santa Clara County since 2019, but Alameda County had a recorded case, in someone who had traveled internationally, in 2024.

A measles case was found in San Mateo County in March, but officials then said, following contact tracing, there was no risk of exposure to the public.

A measles outbreak has been occurring in parts of the US in recent months, which has raised alarms at the same time that Secretary of Health and Human Services Robert F. Kennedy Jr. has been downplaying the necessity of, and raising unscientific questions about, vaccines in general.