(Adriana Zehbrauskas | The New York Times) A sign reading "Wear a mask" is shown in Chinle, Ariz., on the Navajo Nation Reservation, on May 27, 2020. Northern Arizona Healthcare, through its two hospitals in Flagstaff and the Verde Valley, serves a region with 700,000 residents, multiple sovereign Native American nations — including the Navajo Nation — and millions of tourists every year.