An eruption of new coronavirus cases in South Korea, Iran and Chinese hospitals and prisons rekindled concerns Friday about the spread of a deadly disease that has killed more than 2,200 people. The World Health Organization warned nations could face a serious problem if they fail to “hit hard now” against the virus, which has infected more than 75,000 in China and over 1,100 abroad. China has pointed to official figures showing new cases slowing this week as evidence that its drastic containment measures are working, but fresh infections emerged at two Beijing hospitals, and more than 500 others were reported in prisons across the country.