A quarter of the world's population is now living under some form of lockdown. With four hours’ notice, India’s prime minister announced that no one could leave home for 21 days — the most severe step taken anywhere in the war against the coronavirus. — the biggest and most severe action undertaken anywhere to stop the spread of the coronavirus. “There will be a total ban of coming out of your homes,” the prime minister, Narendra Modi, announced on television Tuesday night, giving Indians less than four hours’ notice before the order took effect at 12:01 a.m.