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.

The WHO is an agency of the United Nations tasked with coordinating responses to public health emergencies between nations. The United States is a founding member of the organization, which was set up in 1948 in the wake of World War II.

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.