It's been a mystery from the beginning: why do some people become much sicker - than others with Covid - and why do some people just get a mild case, or don't have symptoms at all? Nearly a year and a half into the pandemic, public health officials are learning more about why that is. Host Arun Rath speaks with Dr. Marcia Goldberg, an infectious disease expert at Mass General Hospital, about a new study that looks at that variability in COVID.