U.S. 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

The move to exit the WHO is historic and controversial. The U.S. has long been the most generous provider of humanitarian assistance.