A winter virus is likely the culprit behind an outbreak of illness that sent dozens of students and adults home early from a five-day educational field trip in Marin County on Wednesday night, a public health official said Thursday.

The affected students and teachers, including some from Danville and Oakland schools, were part of a larger group staying at the Walker Creek Ranch, a campus of the Marin County Office of Education.

“We do know right now that about 67 people total have been ill,” said Anju Goel, deputy public health officer for Marin County. She said environmental health workers were in the process of gathering water and food samples at the site and taking stool samples from sick individuals in an

effort to determine the cause.

A fire engine was sent to the scene on a report of a sick person with flu-like symptoms about 10 p.m. Wednesday. Responders found about 50 students in a weakened condition, according to a fire dispatcher. They were taken home by family members or guardians.

None of those affected, whose main symptoms included vomiting and diarrhea, has been hospitalized or developed more severe illness since Wednesday night, Goel said.

The probable cause of the sickness is norovirus, sometimes called “winter vomiting virus,” she said.