During the press conference where President Trump announced that Vice President Mike Pence would be taking charge of domestic efforts to combat SARS-CoV-2, the new coronavirus spreading internationally, the Centers for Disease Control and Prevention also announced that the outbreak had entered a new phase in the US. The agency reported the first patient infected via “community spread”—which is to say, the patient hadn’t traveled to places where the virus is common and had no known exposure to anyone with Covid-19. WIRED has learned that the patient has been in a northern California hospital for a week, but went undiagnosed until Sunday.

The CDC specified only that the patient was in California; the California Department of Public Health announced that the patient was a resident of Solano County—which, as other reports have pointed out, is the location of one of the Air Force bases being used for quarantines of people who’ve returned to the US from countries with the disease.

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A microbiologist at UC Davis posted to Twitter on Wednesday evening that his university’s hospital had that patient. (He later deleted the tweet.) A letter circulated at UC Davis (signed by David Lubarsky, vice chancellor of human health sciences and CEO of UC Davis Health and Brad Simmons, interim CEO and COO of UC Davis Medical Center) says that the patient has been at the university’s medical center since February 19—a full week ago—and was only diagnosed with Covid-19 on Sunday, February 23. The hospital admitted the patient intubated and on a ventilator, and since health care workers suspected a viral infection they implemented “droplet protection,” keeping on guard against coming into contact with the stuff that comes up in coughing or sneezing.

That gap between admission and diagnosis raises concerns about health care workers being unknowingly exposed at Davis—a potential problem anywhere. Earlier in February, The New York Times reported that more than 1,700 health care workers in China were infected with the coronavirus. Some of them even pleaded for international aid via an article in the journal The Lancet this week. (Worldwide, more than 82,000 people have the disease, and 2,800 have died. The US has 60 known cases.)

The UC Davis memo explains the delay in testing by noting that neither Sacramento County nor the city of Davis’ public health agency performs the test. The hospital had to request the CDC do it. “Since the patient did not fit the existing diagnostic criteria for Covid-19, a test was not immediately administered,” the memo says. On Sunday the CDC did the test, and UC Davis put the patient on more stringent airborne and contact infection control precautions. “Today the CDC confirmed the patient’s test was positive.”

The UC Davis memo also confirms that the hospital has treated other patients infected with Covid-19 and said that the precautions it had taken with the patient all along probably meant “minimal potential for exposure.” Nevertheless, “out of an abundance of caution, in order to assure the health and safety of our employees, we are asking a small number of employees to stay home and monitor their temperature.”

Neither the CDC nor UC Davis Health System returned calls requesting comment.

Despite the president’s attempts to allay concern today, public health officials have been warning to expect community spread of Covid-19. “We expect we will see community spread in this country,” Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said at a press conference on Tuesday.

Now that it’s happening, two more concerns become central. Public health workers are beginning to understand that Covid-19 is quite contagious, though the specific mechanisms aren’t yet established. “Are we potentially missing people who might be sick? This case kind of proves we may be. It doesn’t answer the question of how big the population is,” says Nahid Bhadelia, medical director of the Special Pathogens Unit at Boston University’s National Emerging Infectious Disease Laboratories.