VACAVILLE, Calif. (AP) — Residents of a Northern California community are at the epicenter of what officials are calling a turning point in the spread of the highly contagious coronavirus, as investigators try to retrace the steps of a hospitalized woman they believe to be the first in the U.S. to be infected without traveling internationally or being in close contact with anyone who had it.

As infectious disease experts fanned out in Vacaville, some residents in the city of 100,000 stocked up on supplies amid fears things could get worse despite official reassurances, while others took the ne...