Health officials in Santa Clara County on Friday said that an older adult woman from California is the second case of unknown origin of the novel coronavirus.
Reports from CNN suggest she is thought to be the third case of community spread in the US, another coronavirus infected patient from California is rumoured to be the second.
The infected woman was hospitalized for a respiratory illness and her infectious disease doctor on Wednesday requested a coronavirus test.
The director of US public health department Dr Sara Cody, said that the patient did not have any relevant travel history or exposure to another known patient.
"This new case indicates that there is evidence of community transmission, but the extent is still not clear," Cody said.
She added, "I understand this may be concerning to hear, but this is what we have been preparing for. Now we need to start taking additional actions to slow down the spread of the disease."
The virus is here present at some level, but we still do not know to what degree, the doctor explained.
The Centre for Disease Control (CDC) this week updated its testing criteria after a woman from California was not tested for days after being hospitalized. She is reported to be in critical condition.
Health officials say they have not been able to trace where she contracted the virus.
"This does appear to be a person who genuinely did acquire their illness in the community," Solano County Public Health Officer Dr Bela Matyas
There are 65 confirmed cases of coronavirus in the US, according to health officials, but no deaths.
In Nebraska, a team of researchers are working to evaluate how effective the antiviral drug remdesivir would be in treating people diagnosed with the virus, the National Institutes of Health said.
The clinical trial, taking place at the University of Nebraska Medical Center, in Omaha, will test the drug first on an American who was evacuated from the Diamond Princess cruise ship.
So far, there have been more than 84,400 confirmed cases of coronavirus across the world and at least 2,919 deaths -- the vast majority in China, according to CNN.