Amy Watson, 47, is one of those patients. She's had a fever, she said, for more than 100 days. She was diagnosed with COVID-19 in April, about a month after her symptoms — cough, congestion and extreme fatigue — began. Now, those symptoms have evolved into weeks of low-grade fever and a burning sensation under her skin. Watson's illness was never severe enough to warrant hospitalization. Instead, her symptoms have lurked in the background, never fully resolving Dr. Jessica Dine, a lung doctor at Penn Medicine in Philadelphia, said she began noticing a subset of COVID-19 patients whose symptoms lingered...