California public health officials are warning of a possible measles exposure spanning multiple counties after an infected and unvaccinated passenger traveled through the state via Amtrak trains and thruway buses.

The warning comes as the US faces its largest measles resurgence in more than three decades, fueled largely by growing vaccine hesitancy.

The California department of public health said the traveler passed through at least six counties – Alameda, Butte, Kern, Los Angeles, Sacramento and San Joaquin – while making multiple transfers.