Officials in Northern California are warning Bay Area Rapid Transit commuters for a second time this month that they may have been exposed to measles on trains.

The San Francisco Chronicle reports (http://bit.ly/1Evn7HD) that an infected person commuted from the Millbrae station to the Civic Center Station on Friday from about 4:30 to 5 p.m.

Public health officials say the risk to other riders is low, and almost negligible for anyone who has been immunized. Still, the virus can live for up to two hours on a surface or in an airspace where the infected person coughed or sneezed.

Contra Costa County public health officials reported earlier this month that someone diagnosed with measles rode BART trains between Lafayette and San Francisco on Feb. 4, 5 and 6.