California health officials warned residents of possible measles exposure in six counties after an unvaccinated passenger traveled on Amtrak trains and connecting buses earlier this month.

The California Department of Public Health said the individual arrived in Los Angeles from Chicago on September 12 and continued through Bakersfield, Stockton, Oroville, Sacramento and Emeryville before departing the state for Denver on September 14. The affected counties include Alameda, Butte, Kern, Los Angeles, Sacramento and San Joaquin.

Anyone who was on those specific trains or buses, or present at the stations during the listed arrival and departure times, may have been exposed. Measles is highly contagious and can remain in the air for up to two hours after an infected person leaves an area.

The passenger later required hospitalization with the disease. Officials are working with county health departments and federal partners to notify potential contacts.

Nationwide, more than 3,400 measles cases have been reported in 2026, the highest total since 1991. California has recorded 59 cases across 13 counties so far this year, with 94 percent of patients unvaccinated or of unknown vaccination status.

Assistant State Public Health Officer Dr. Rita Nguyen emphasized that the MMR vaccine provides strong protection and urged Californians to check their vaccination status. Symptoms of measles include fever, cough, runny nose, red eyes and a rash, and complications can be severe, especially for young children.

Public health experts have linked rising cases to pockets of lower vaccination rates in some communities. The warning comes amid broader national increases in the vaccine-preventable illness.