New research on how some people had a level of Covid immunity before the pandemic started. Blood samples showed hospital staff being monitored in the first wave already had protective ‘killer’ T-cells probably from exposure to other viruses related to the one that has swept the globe. The difference between antibodies to an infection and antibodies caused by a vaccine. And the extraordinary story of a woman who rid her body of HIV. Presenter: Claudia Hammond Producer: Erika Wright (Picture: Microbiologist studying coronavirus. Photo credit: Janiecbros/Getty Images)