The California patient didn't travel anywhere known to have the virus, the US Centers for Disease Control and Prevention said. And the patient wasn't exposed to anyone known to be infected.
The Solano County resident was admitted to UC Davis Medical Center last week but wasn't tested for coronavirus until Sunday, according to a letter sent Wednesday to UC Davis staff and obtained by CNN.
UC Davis had requested coronavirus testing for the patient, but the patient "did not fit the existing CDC criteria" for such testing, the letter said.
The mysterious nature of this case is significant, said Dr. Dean Blumberg, an infectious disease specialist at UC Davis Medical Center.
While it's not clear who the UC Davis patient got the virus from, "that other person probably exposed other people," Blumberg said.
"And you have to realize that this virus is so new, that none of us have any immunity to it. So, anybody who's exposed is at high risk of getting infected with this."
The US has 60 confirmed cases of novel coronavirus, Health and Human Services Secretary Alex Azar said.
The UC Davis patient was transferred from another California hospital on February 19 with a suspected viral infection, the university hospital's letter said.
UC Davis said it requested CDC testing for COVID-19, the formal name of the virus, because neither Sacramento County nor the California Department of Public Health were conducting testing.
"Since the patient did not fit the existing CDC criteria for COVID-19, a test was not immediately administered," the letter says. "UC Davis Health does not control the testing process."
The CDC ordered testing on Sunday and results came back positive Wednesday, the letter says.
There has been concern over the tests and their availability.
The US Food and Drug Administration has authorized a two-step test for the novel coronavirus that will allow labs other than the CDC to test for the virus, Azar said Thursday.
Previously, the process for testing involved three steps. The third step that had been causing inconclusive results, Azar said.
As of Wednesday afternoon, "the FDA authorized the use of those tests by using just the first and second step (to) provide a definitive diagnostic," Azar said.
"Forty labs are qualified to already be doing that," he said.
By this weekend, 93 labs across the US are expected to have testing capabilities, Azar said.
And as many as 70 companies are vying to develop a "bedside diagnostic" test for hospitals use.
In the early weeks of the outbreak, the CDC was the only facility in the US that could test for the virus.
If the California case does turn out to be community spread, hospital staff will need to alter the way they evaluate sick patients, one health care executive said.
Hospitals receiving patients with flu-like symptoms have been asking people to detail their travel history or exposure to anyone who has recently traveled, said Dr. Amy Compton-Phillips, chief clinical officer and executive vice president of Providence St. Joseph Health.
"Now, we're going to have to be thinking: How do we change that strategy? Who all do we need to be testing for coronavirus? Right now, during flu season, that would be a lot of people, and so we need to figure out how we can scale up the testing and the screening," she said.
Providence St. Joseph Health operates 51 hospitals and more than 1,000 clinics across the country.
"We are asking the American public to work with us to prepare in the expectation that this could be bad," she said.
"There's a very limited number of them," she said.
There have been at least 82,056 confirmed cases worldwide and 2,800 deaths -- the vast majority in China.
The drug being tested in the trial, remdesivir, was previously tested in humans for Ebola and in animals for MERS and SARS.
There are also clinical trials of the drug going on in China, the NIH said, and the National Institute of Allergy and Infectious Diseases developed the American study to match those studies.
The first participant in the Omaha trial is an American who was evacuated from the Diamond Princess cruise ship.
Participants will receive 200 milligrams of remdesivir intravenously when they're enrolled and another 100 while they're hospitalized for up to 10 days in total. A placebo group will receive a solution that resembles remdesivir but contains only inactive ingredients, the NIH said.
CNN's Amanda Watts and Shelby Lin Erdman contributed to this report.