A seriously ill Solano County resident appears to be the nation’s first case of coronavirus infection from an unknown source, fueling concern that the virus could spread undetected in the general population.

The diagnosis was delayed, potentially exposing others to infection, because a federal test was not promptly conducted when the patient arrived at UC Davis on Wednesday Feb. 19, despite the medical center’s request. The test was conducted four days later, on Sunday Feb. 23, and results returned on Wednesday Feb. 26.

Unlike other cases, the person did not recently return from a foreign country and is not known to have had contact with a person who was sick or infected, according to the U.S. Centers for Disease Control.

Such community transmission is worrisome because a virus is much harder to control once it spreads in offices, schools, medical offices and other public sites. The case differs from others in the U.S., where the infected persons were known travelers who were quickly and safely quarantined, or persons in close contact with those travelers.

The patient was transferred from another Northern California hospital, where healthcare workers also were unaware of the dangerous infection.

The patient is now being treated in the Intensive Care Unit at UC Davis Medical Center, according to infectious disease specialist Dr. Bennett Penn, assistant professor of microbiology at UC Davis.

Health officials are tracking down contacts of the patient, as well as investigating whether the person was unknowingly exposed to a returned traveler who was infected. Solano County is home to Travis Air Force Base, where many travelers have been quarantined.

According to a statement from two UC Davis officials, the test was delayed because the patient did not fit into the CDC’s two criteria: a history of travel to China or close contact with a coronavirus patient.

“Upon admission, our team asked public health officials if this case could be COVID-19,” the name of the respiratory disease, according to a memo to UC Davis medical staff from David Lubarsky, vice-chancellor of human health services, and Brad Simmons, interim CEO of UC Davis Medical Center. “We requested COVID-19 testing by the CDC, since neither Sacramento County nor (the California Department of Public Health) is doing testing for coronavirus at this time.”

“Since the patient did not fit the existing CDC criteria for COVID-19, a test was not immediately administered,” they said. “UC Davis Health does not control the testing process.”

A small number of medical center employees have been asked to stay home and monitor their temperatures, according to the statement. But “because of precautions we have had in place since this patient’s arrival, we believe there has been minimal potential for exposure here at UC Davis Medical Center,” the email said.

This case means that the U.S. joins other nations where the virus is appearing without a known source of exposure, such as Hong Kong, Italy, Iran, Singapore, South Korea, Taiwan and Thailand.

“I thought it’d be a while before this happened, but looks like it’s time to buckle up,” Tweeted UC Davis microbiologist Penn.

The news came as President Donald Trump spoke at an evening news conference in which he was optimistic about the administration’s efforts to prevent the spread of the disease, predicting cases would drop “down to zero in a few days” but vowing the U.S. is ready to handle an outbreak, should it come.

There have been two previous instances of person-to-person transmission in the United States — one in San Benito County and the second in Chicago — but both of those cases were after close, prolonged interaction with an infected family member who returned from Wuhan, China.

Not counting infections among passengers of the ill-fated Diamond Princess cruise ship, now quarantined at Travis Air Force Base, California has had seven travel-related cases, one close contact case — and now one community transmission, according to the state Department of Public Health.

Does this mean that the coronavirus outbreak could pose a threat to you and your family?

Yes, no and maybe.

Health officials insist the risk to Americans remains low. Hundreds of people who were airlifted from China to the U.S. last month were given a clean bill of health and allowed to leave quarantine. About 60 people have tested positive — most of them the cruise ship passengers who were brought back to the U.S. from Japan earlier this month.

The vast majority of confirmed cases are considered mild, involving mere cold-like symptoms to mild pneumonia. But the illness can be catastrophic for five percent of patients, particularly the elderly.

To make sense of the fast-moving world of coronavirus science and policy, we’ve collected the latest information from U.S. and global press briefings, virologists and research studies.

Q: Why can’t my doctor diagnose it?

A: The symptoms of coronavirus look a lot like every illness that circulates this time of year. So it’s tough for doctors to make a diagnosis solely on the basis of the symptoms.

The only way to find out is to be tested. The California Department of Public Health’s state laboratory in Richmond is using the CDC’s detection test on people repatriated from overseas who are suspected to be infected, but there have been undisclosed problems with it — and that’s causing a delay. Tests that are positive must be shipped to Atlanta for confirmation.

It’s important to be fast, but it’s more important to be accurate. The CDC is fixing its test and soon will send out a new version. When commercial tests are available, there will be greater efficiency.

— California Department of Public Health and Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases

Q: If I’m exposed, how long before I develop symptoms?

A: The period between exposure and symptoms is 5.2 days on average, but varies greatly among patients. A new analysis still supports a 14-day medical observation period for people exposed to the pathogen.

— New England Journal of Medicine

Q: How sick would I get?

A: Across China, about 80% of cases are mild, involving mostly cold-like symptoms to mild pneumonia. Fourteen percent of cases have been “severe,” involving serious pneumonia and shortness of breath. Another 5 percent of patients developed critical respiratory failure, septic shock and organ failure.

Patients 80 years or older are most at risk, with 14.8 percent of them dying. Deaths occurred in every age group except in children under the age of nine.

For people with mild disease, recovery time is about two weeks, while people with severe disease recover within three to six weeks.

— Chinese Center for Disease Control and Prevention

Q: Why are some people hit so hard?

A: Experts think it depends on a person’s immune response. It seems to trigger an overproduction of immune cells that flood into the lung, causing pneumonia.

— Angela Rasmussen, a virologist at Columbia University’s Mailman School of Public Health

Q: How easily does it spread?

A: Each infected person infects an average of 2 to 3.1 other people. That’s higher than the seasonal flu (around 1.3) but much lower than the measles (12 or higher).

— Maia Majumder, Boston Children’s Hospital

Q: Can I spread it if I’m not sick?

A: Perhaps. In two patients, German researchers were able to isolate the virus from the nose and throat even before they showed any symptoms or had very mild symptoms.

— New England Journal of Medicine

Q: Does a mask help or hurt?

A: The CDC does not recommend wearing masks. Health care workers who care for care for patients with coronavirus should take all precautions. So should household members or close contacts of a known or suspected infected person. But in terms of day-to-day interactions, hand washing is far more effective.

A surgical mask is designed to keep the surgeon’s germs off the patient.

An ‘N95’ respirator mask is better in terms protecting you but can be difficult to fit correctly to the face. If a mask is ill-fitting, the virus can find a way in. If you wear that mask, it should be specially fitted.

— Virologist Dr. Warner C. Greene, professor of medicine, microbiology and immunology at UC San Francisco

Q: Can local hospitals treat patients?

A: Patients who require hospitalization can be cared for in any community hospital that is reasonably well prepared. Almost all hospitals already have trained for a flu pandemic — and should start practicing for coronavirus care now.

— Dr. Eric Toner of the Johns Hopkins Center for Health Security.

Q: What are my chances of dying?

A: The death rate — the percentage of known infected people who die — is about 2% in China’s Hubei province, where the virus first started, and 0.7% in other parts of China. The risk of death in those over age 80 is high, at 14%. For children and young people, it is very low.

That is comparable to the death rate of the 1918 Spanish Flu pandemic. It’s much higher than the 0.1 percent fatality rate for seasonal flu. But it’s lower than SARS virus, at 9-10%, and much lower than the MERS outbreak, at 36%.

— Dr. Bruce Aylward, World Health Organization

Q: There are no approved drugs to treat coronavirus. Are any being tested?

A: Yes. Trials already have started. The drug remdesivir, made by Gilead Biosciences of Foster City, is being tested on sick Americans at the University of Nebraska Medical Center in Omaha. While small, this is a rigorous study; it is ‘blinded,” with some subjects getting the drug and others getting a placebo. It is also being tested in China.

— Dr. Anthony Fauci, National Institutes of Allergy and Infectious Disease

Q: There’s no vaccine. What’s in the pipeline?

It is likely to be a year or 18 months before the vaccine is widely available. But things are moving quickly.

The company Moderna Therapeutics already has already shipped some doses of an RNA vaccine to the NIH. Those could be given to volunteers in a safety test starting as soon as April.

— Dr. Anthony Fauci, National Institutes of Allergy and Infectious Disease

Q: What’s happening in China?

A: Astonishingly, the number of new cases is declining, thanks to the country’s aggressive lockdown. The Chinese epidemic peaked between Jan. 23 and Feb. 2.

Fourteen other countries have not reported a new case in more than a week — and even more important, nine countries have not reported a case for more than two weeks.

— Dr. Tedros Ghebreyesus, World Health Organization

Q: Is it a pandemic?

A: It meets two of three criteria for a pandemic: illness resulting in death and sustained person-to-person spread. It’s moving closer but has not yet met the third criteria: worldwide spread.

— Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases

Q: Where did it start? Q: How does it make you sick? Q: Can be become immune? Answers to these questions and more: https://www.mercurynews.com/2020/02/11/coronavirus-the-8-big-questions-that-scientists-are-asking/

John Woolfolk contributed to this article.