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.

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.

“For over seven decades, WHO and the USA have saved countless lives and protected Americans and all people from health threats. Together, we ended smallpox, and together we have brought polio to the brink of eradication,” WHO said.