A Placer County man who died last week from coronavirus almost certainly became infected before he boarded a cruise ship from San Francisco to Mexico on Feb. 11, suggesting that he was exposed to the virus in the community and it has been circulating in parts of California for longer than previously reported, according to a cruise ship doctor.

Dr. Grant Tarling, chief medical officer for Carnival Corporation, which operates the Grand Princess cruise ship on which the Placer County resident had traveled, said that the man was seen by a ship doctor on Feb. 20 for serious respiratory problems. The man told the doctor that he had been feeling unwell for six or seven days, Tarling said.

That would mean the man first had symptoms on Feb. 13 or Feb. 14 — two or three days after boarding the ship in San Francisco. People infected with the coronavirus typically show symptoms no sooner than four days after exposure, studies have found.

“That would have put his exposure period before he joined the vessel,” Tarling said. “So we believe his illness was probably community acquired somewhere in California before he joined the ship.”

So far 30 people who were on the Grand Princess on one of two recent voyages have tested positive for coronavirus. Tarling believes that the Placer County man is the “index case” at the center of the outbreak.

The first reported case of community infection in the United States was Feb. 26, involving a Solano County woman who tested positive while she was hospitalized in Sacramento with serious illness. If the Placer County man was infected by a community exposure, that would mean the virus has been circulating in California for two weeks longer than earlier reports.

Some infectious disease experts believe that the virus probably has been circulating to some degree in the United States since early February or even January.

The timeframe of the Placer County man’s illness is not confirmed, and it’s still possible he became infected while on the ship. It’s possible that he misreported the timing of his symptoms. Also, some studies have found that the incubation period for coronavirus can be as short as one or two days.

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com