The first-floor classroom is empty inside the Boston University School of Medicine on a gray and chilly May morning in the South End. The whiteboard is marked up from earlier instruction and the only sound is the hum of fluorescent lights overhead. With an audience of one, and from behind her blue mask, Nahid Bhadelia slips on one hat after another, pivoting seamlessly from healthcare policy wonk to infectious diseases physician to MED associate professor to expert researcher in highly communicable diseases.