The Solano County patient did not recently travel out of the U.S. and is the first known person to contract the illness through person-to-person spread.
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The woman is one of 33 Californians who've contracted the disease since the outbreak first began.
"The Centers for Disease Control and Prevention (CDC) has confirmed an infection with the virus that causes COVID-19 in California in a person who reportedly did not have relevant travel history or exposure to another known patient with COVID-19," the CDC said in a statement.
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According to Aimee Brewer, president of NorthBay Healthcare Group, the patient stayed at VacaValley hospital in Vacaville for three days -- but did not fit federal health criteria for the coronavirus, so she was not tested.
In a statement, Brewer said the woman's conditions worsened, prompting her to be transferred to UC Davis Medical Center in Sacramento.
She arrived at UC Davis Medical Center on Feb. 19.
After learning the patient tested positive for the coronavirus, the hospital began tracing those at the VacaValley who may have had contact with the woman.
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Employees who were considered moderate or high risk were asked to stay home, Brewer said.
The number of employees sent home is less than 100, "but in the dozens," a hospital spokesman told ABC7.
Health officials said Thursday in a press conference that the Solano County patient was in the community for several days before initially showing symptoms.
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They are now working to retrace her steps and isolating the people who may have come into contact with her.
"We knew this was inevitable," Gov. Gavin Newsom said during a Thursday press conference. "We're meeting this moment."
In Sacramento, the governor also addressed a shortage of coronavirus testing kits, saying California has 200 kits at this time, which he said is "inadequate."
The CDC intends to provide California with more kits in the coming days, the governor said.
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More than 8,400 people in California are being monitored for the coronavirus, he said.
"We knew this was inevitable as it relates to the nature of these viruses," Newsom said.
Despite the newest case originating in Solano County, health officials reiterated the risk to the public is low.
Amid concerns over the coronavirus outbreak, school districts in Vacaville and Fairfield have also addressed protocol to keep students and staff safe.
Travis Unified School District said its making sure to keep schools sanitized.
"We can assure you that our staff is doing everything in their power to keep school sites disinfected and clean for all students and staff. It is our standard protocol to perform routine surface cleaning throughout our school sites and offices," the district said in a statement.
Fairfield-Suisun Unified School District addressed outbreak concerns similarly, encouraging students to stay home if sick and to wash their hands frequently.
"We understand the Novel Coronavirus is causing concern for many of our parents," the district said. "Please keep in mind that this is a virus, and it is not specific to any single race or ethnicity. It is important to speak with your children about cultural sensitivity during this challenging time."
The news of the latest patient in Solano County comes as others are being quarantined at Travis Air Force Base.
Those patients were passengers on the Diamond Princess cruise ship in Japan.
Health officials urge the public that the risk of the virus remains low and most who contract coronavirus will not require hospitalization.