The calls kept coming late Saturday night, rousing Mary Krebs from sleep again and again at her farmhouse 15 miles outside Dayton, Ohio.

As the only doctor at a local community health clinic for low-income patients, Dr. Krebs was on call and advising a flurry of anxious patients who were worried they might be ill with Covid-19, the respiratory disease caused by the novel coronavirus.

She sent one with trouble breathing to the emergency room. She told others without significant symptoms to rest, drink plenty of fluids, avoid others and call back if anything changed.

What she didn’t tell them: She was also sick. Dr. Krebs had diagnosed herself with Covid-19, based on her telltale symptoms of fever, shortness of breath, dry cough and exposure to a patient who had tested positive. She was quarantined at home.

Every time the phone rang, the 40-year-old family physician would sit up, open her laptop and get to work.