The World Health Organization is shaped by its members: 194 countries that set health priorities and make agreements about how to share critical data, treatments, and vaccines during international emergencies.

Public health experts say the United States’ departure could cripple the WHO’s operations or leave an opening for China to assume greater control over the agency.

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.