On the pandemic’s first anniversary, some COVID-19 survivors are still struggling with puzzling symptoms. Now, scientists are racing to solve what they consider a true medical mystery. Long-term COVID-19 affects an uncertain number of survivors in a baffling variety of ways. The government is spending $1 billion in an unprecedented effort to find answers. Is it a condition unique to COVID-19, or a variation of the syndrome that can occur after other infections? Or could some symptoms be a physical reaction to the enormous upheaval of this past pandemic year? Researchers are seeking disease markers, treatments and cures.