A security guard leaves NorthBay VacaValley Hospital, where a woman diagnosed with coronavirus previously sought treatment, on Thursday, Feb. 27, 2020, in Vacaville, Calif. Public health officials are retracing the steps of the woman, believed to be the first person in the U.S. to contract the highly contagious coronavirus without traveling abroad or coming in close contact with an international traveller. (AP Photo/Noah Berger)