Have we been vastly underestimating the number of people who have been infected with Covid-19 and correspondingly overestimating its mortality? No one knows because we don’t know just how widespread this infection is in the population at large. But a leaked document from the US Department of Homeland Security suggests that the US government, at least, is working on the assumption that the virus is a lot harder to contain – but a lot less deadly – than is widely assumed.