Covid-19: Why are people suffering long-term symptoms?

Weeks and months after having a confirmed or suspected Covid-19 infection, many people are finding they still haven't fully recovered. Emerging reports describe lingering symptoms ranging from fatigue and brain-fog to breathlessness and tingling toes. So why does Covid-19 cause lasting health problems? Ian Sample discusses some of the possible explanations with Prof Danny Altmann, and finds out how patients might be helped in the future