The 47-year-old woman couldn’t shake her cough and shortness of breath, baffling Veronique Nussenblatt, an infectious disease specialist at the National Institutes of Health (NIH). Her patient had been hospitalized at NIH with COVID-19 in spring 2020; as summer turned to fall and fall to winter, the woman should have mostly recovered. But she continued to need supplemental oxygen at home. “Sometimes she felt better, sometimes she felt worse,” Nussenblatt says.