Polio had been considered eliminated from the U.S. since 1979, when the last known case of the original strain of the polio virus was detected, while the global vaccination campaign has decreased cases by 99.9 percent. But the virus now seems to be gaining a foothold again in some countries. Dr. Yvonne Maldonado of the Stanford School of Medicine joins William Brangham to discuss. PBS NewsHour is supported by - https://www.pbs.org/newshour/about/funders