We get to the bottom of a medical mystery – what causes multiple sclerosis? A series of studies have compellingly pointed the finger at the virus behind glandular fever. We see if they stack up and assess what it means for the future of preventing and treating MS. Then nearly two years since the World Health Organization described Covid as a pandemic, James chats to Dr Maria van Kerkhove, who is the WHO's technical lead for its response to Covid, about the successes and failures of the past two years and where we’re all heading next? Producer: Andrew Luck-Baker