The Centers for Disease Control and Prevention is investigating what could be the first case of novel coronavirus in the United States involving a patient in California who neither recently traveled out of the country nor was in contact with someone who did.

“At this time, the patient’s exposure is unknown. It’s possible this could be an instance of community spread of COVID-19, which would be the first time this has happened in the United States,” the CDC said in a statement. “Community spread means spread of an illness for which the source of infection is unknown. It’s also possible, however, that the patient may have been exposed to a returned traveler who was infected.

The individual is a resident of Solano County and is receiving medical care in Sacramento County, according to the state Department of Public Health.

The CDC said the “case was detected through the U.S. public health system — picked up by astute clinicians.”

Officials at UC Davis Medical Center expanded on what the federal agency might have meant by that in an email sent Wednesday, as reported by the Davis Enterprise newspaper.

The patient arrived at UC Davis Medical Center from another hospital Feb. 19 and “had already been intubated, was on a ventilator, and given droplet protection orders because of an undiagnosed and suspected viral condition,” according to an email sent by UC Davis officials that was obtained by the Davis Enterprise.

The staff at UC Davis requested COVID-19 testing by the CDC, but because the patient didn’t fit the CDC’s existing criteria for the virus, a test wasn’t immediately administered, according to the email. The CDC then ordered the test Sunday, and results were announced Wednesday. Hospital administrators reportedly said in the email that despite these issues, there has been minimal exposure at the hospital because of safety protocols they have in place.

A UC Davis Health spokesperson declined Wednesday evening to share the email with The Times.

Since Feb. 2, more than 8,400 returning travelers from China have entered California, according to the state health department. They have been advised to self-quarantine for 14 days and limit interactions with others as much as possible, officials said.

“This is a new virus, and while we are still learning about it, there is a lot we already know,” Dr. Sonia Angell, director of the California Department of Public Health, said in a statement. “We have been anticipating the potential for such a case in the U.S., and given our close familial, social and business relationships with China, it is not unexpected that the first case in the U.S. would be in California.”

It is not clear how the person became infected, but public health workers could not identify any contacts with people who had traveled to China or other areas where the virus is widespread. That raises concern that the virus is spreading in the United States, creating a challenge for public health officials, experts say.

“It’s the first signal that we could be having silent transmission in the community,” said Lawrence Gostin, director of the World Health Organization Collaborating Center on National and Global Health Law. “It probably means there are many more cases out there, and it probably means this individual has infected others, and now it’s a race to try to find out who that person has infected.”

On Tuesday, the CDC offered its most serious warning to date that the United States should expect and prepare for the coronavirus to become a more widespread health issue.

“Ultimately, we expect we will see coronavirus spread in this country,” said Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases. “It’s not so much a question of if, but a question of when.”

According to the CDC’s latest count Wednesday morning, 59 U.S. residents have tested positive for the new strain of coronavirus — 42 of whom are repatriated citizens from a Diamond Princess cruise. That number has grown by two since Messonnier’s last count Tuesday, although the CDC was not immediately available to offer details on the additional cases.

More than 82,000 cases of coronavirus have been reported globally, and more than 2,700 people have died, with the majority in mainland China, the epicenter of the outbreak.

But public health leaders have repeatedly reminded residents that the health risk from the novel coronavirus to the general public remains low.

“While COVID-19 has a high transmission rate, it has a low mortality rate,” the state Department of Public Health said in a statement Wednesday. “From the international data we have, of those who have tested positive for COVID-19, approximately 80% do not exhibit symptoms that would require hospitalization. There have been no confirmed deaths related to COVID-19 in the United States to date.”

CDC officials have also warned that although the virus is likely to spread in U.S. communities, the flu still poses a greater risk.

Gostin said the news of potential silent transmission does not eliminate the possibility of containing the virus in the U.S. and preventing an outbreak.

“There are few enough cases that we should at least try,” he said. “Most of us are not optimistic that that will be successful, but we’re still in the position to try.”