SAN FRANCISCO — A San Francisco bus driver was beaten with a bat by a passenger who refused to wear a face mask, police said.

Three young men without masks got on a public bus Wednesday afternoon and the driver repeatedly asked them to don face coverings, police said.

A city health order requires masks to be worn on public transport, an effort to slow the spread of the coronavirus.

When the driver pulled over to escort the men off the bus, one pulled out a wooden bat and hit the driver several times, police said.

The men fled and haven't been arrested.

The driver was left with injuries that were not life-threatening and is recovering, police said.

For most people, the new coronavirus causes mild or moderate symptoms, such as fever and cough that clear up in two to three weeks. For some, especially older adults and people with existing health problems, it can cause more severe illness, including pneumonia and death.