Ooh, that’s a big one,” Donald Trump said Monday as he signed an executive order – one of dozens during his first hours as president – to withdraw the United States from the World Health Organization.

President Donald Trump used one of the flurry of executive actions that he issued on his first day back in the White House to begin the process of withdrawing the U.S. from the World Health Organization for the second time in less than five years — a move many scientists fear could roll back decadeslong gains made in fighting

Public health experts say U.S. withdrawal from the W.H.O. would undermine the nation’s standing as a global health leader and make it harder to fight the next pandemic.