Three years ago the German film director Christian Petzold was promoting his mermaid drama Undine in Paris when he and its star, Paula Beer, noticed Emmanuel Macron on television. The announcement was unambiguous: lockdown in effect, immediately. The new social-distancing protocols came too late for Petzold and Beer. They were already infected with Covid. “I think we were the first patients in Germany,” the film-maker says. “Laboratory technicians were calling and asking, ‘Can you give us your blood?’ I had to lie down in bed for weeks, having fever dreams. I had a script based on a dystopian story by...