An investigation into how a Contra Costa County resident contracted the county’s first case of coronavirus continued Wednesday afternoon, as health officials worked to determined the level of exposure others may have had to the virus.

“We’re basically following the protocol,” said Contra Costa County Health Department spokesman Will Harper. “We’re investigating where the patient has been and the people the person has been in contact with.”

Officials said the stricken resident is a man who had underlying health issues; they declined to give his age. He remained in critical condition at an undisclosed hospital in the county on Wednesday.

It is the first case of COVID-19 in Contra Costa County involving a person who wasn’t transferred to a hospital from somewhere else, Harper said.

The county already has treated three other people with coronavirus, Harper said. Those patients were transferred to undisclosed hospitals in the county from Travis Air Force Base last week, Harper said.

They had been taken for quarantine after being evacuated from the Diamond Princess cruise ship. Two of those three were later moved out of the county, Harper said. The third is recovering.

County officials have not determined where or how this case began.

“A lot depends on where he was,” Harper said, when asked how concerned the county may be that he infected others. “On a place like a BART train, you can be shoulder-to-shoulder with others. In a place like a grocery store, you aren’t necessarily that close together.”

The news in Contra Costa County came on the same day that Santa Clara County announced two new cases in people without a known exposure risk. Health officials there also warned residents 50 and over and those with serious health conditions to avoid large public gatherings.

“The risk increases with age,” said Dr. Sara Cody, Santa Clara County’s health officer. “We are recommending people at higher risk avoid mass gatherings.”

At a Tuesday news conference in Contra Costa County, Dan Peddycord, director of public health, and Ori Tzvieli, medical director for the public health division, said the patient is an adult male with underlying health conditions who was initially hospitalized with respiratory and flu-like symptoms. The man is currently isolated in an area hospital.

Harper said the public should be more vigilant than ever against the virus. People are urged to wash their hands for at least 20 seconds several times a day and to avoid touching their face.

“The big one is if you’re sick, don’t go to work or school,” Harper said. “We’ve also advised employers to review their sick leave policies, because people feel obligated to be at work. These days, people can often get a lot of their work done from home, so if there’s any doubt at all, they should stay home.”

Check back for updates. Staff writer Lisa Krieger contributed to this story