Dr. Chris Spitters, health officer of the Snohomish Health District, said he suspected that the teenager probably got sick from someone with a mild form of the illness who did not seek care. “I think we’re going to find as time goes on that there’s more of that,” Dr. Spitters said.

In Oregon, a state that had not reported any previous cases of coronavirus, officials said an employee of Forest Hills Elementary School in Lake Oswego appeared to have contracted the virus more than a week ago.

Oregon health officials said their public health laboratory, which began testing samples on Friday, had detected the positive case; the sample will also be tested at the Centers for Disease Control and Prevention for confirmation, as will the new cases from Washington and California.

In California, a woman from Santa Clara County with chronic health conditions who was hospitalized for a respiratory illness was reported on Friday to have tested positive for infection with the virus. Two days earlier, officials in Solano County, Calif., 90 minutes from Santa Clara County, had announced the first case in the nation with no clear ties to any previous case or overseas travel.