Cases of the coronavirus in prisons and jails across the United States have soared in recent weeks, even as the overall daily infection rate in the nation has remained relatively flat. The number of prison inmates known to be infected has doubled during the past month to more than 68,000. Prison deaths tied to the coronavirus have also risen, by 73 percent since mid-May. By now, the five largest known clusters of the virus in the United States are not at nursing homes or meatpacking plants, but inside correction institutions