USAA woman this week infected with unexplained nCoV in Solano County, California. State officials yesterday detected a similar case in Santa Clara county.

The Centers for Disease Control and Prevention (CDC) cannot determine where these two patients have been infected with nCoV. They have not recently come to an epidemic area, had no contact with people who have been abroad or who have been confirmed to be infected with the virus. These two patients are also unrelated.

The US has detected 63 cases of nCoV, but most of them have come back from Wuhan, who have been abroad and passengers aboard Diamond Princess. Officials say two "mysterious" cases in California are the first signs that nCoV may be spreading in the US community.

"We are more worried today than yesterday," said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center. Now we have to ask the question: "How far has the nCoV really spread?".

"This case signals to us that it's time to change the way we deal with nCoV," said Sara Cody, a public health official in Santa Clara County, California. "Now we need to deploy more public health tools."

President Donald Trump said on Monday during a news conference that the risk of nCoV in the US was "very low". However, Nancy Messonnier, CDC's top official, warned on February 25 that the impact of nCoV on the US could be huge, causing serious disruptions in everyday life. She said the widespread epidemic in the US was "inevitable" and urged people to take preventive measures.

"The question is no longer whether it will happen in the US, it's just a question of when and how many people will get sick," she said.

Experts say health officials in two counties in California need to review samples of people who have been hospitalized or died of respiratory disease in recent weeks.

Santa Clara County is the "heart" of Silicon Valley, where Google and Apple are based. The district also includes San Jose, the 10th largest city in the United States. Cody advises schools and businesses to begin planning for disinfection and coping in the event of a major outbreak.

The mysterious "nCoV" patient in Santa Clara is an elderly woman with a chronic illness. County officials are rushing to identify those grandmothers who have been in contact.

In Solano County, where the first unidentified case was discovered, officials on Monday declared a medical emergency. Bela T. Matyas, a public health officer in Solano county, said officials were monitoring dozens of people the patient had been in contact with in recent weeks and asked them to quarantine at home. People with symptoms will be tested.

Solano County is home to Travis Air Force Base, the facility that isolate hundreds of Americans from Asia. "A few weeks ago, when I saw Solano appearing on every news of Travis base and the quarantine, I thought that sooner or later there would be a problem," Cricket Kanouff, president of the Pena Adobe Historical Society, said.

A number of quarantined people at Travis base were positive for nCoV. However, officials stressed that there was no sign that the "mysterious" case had contacted anyone at the base.

Outside the air base, everyone seemed quite calm. Pilots in uniforms shook hands in the nearby fast food restaurant. Beverly Garske, a pedestrian near the base, said: "To me, it's just like the flu."

Garske feels the virus is "closer to her" but believes she will not be infected. "My husband and I don't go out much, often we just go out fishing," she said.

Nern Lewis, living near Travis air base, is also not afraid. "I was mostly in the house," she said. But some of her relatives are increasingly worried. Lewis originally planned to return to Thailand to visit his sister, but had to postpone the trip. "I'm afraid I can't come back," she said.

The patient in Solano County was initially treated at NorthBay VacaValley Hospital in Vacaville and then ambulance taken to Davis Medical Center at the University of California at Sacramento. Some staff at the hospital worried about the risk of infection.

"I'm worried this will affect my baby," said Vickie Poncalo, a cashier at the hospital's cafe district, who is seven months pregnant. "Should we wear a mask and worry about people walking around?" She said. "Should I continue working?"

In the gift shop, Madeline Daniels is thinking about ways viruses can spread. "Money is the dirtiest thing," she said.

Peter L. Beilenson, director of the Sacramento County Health Department, said officials were preparing to cope with the risk of a widespread epidemic in Northern California. "Almost certainly there will be quite a lot of people who are positive for nCoV," Beilenson said.