CDC ordered to immediately stop collaborating with WHO after Trump begins process for US withdrawal

According to the memo, the stop-work policy applied to "all CDC staff engaging with WHO through technical working groups, coordinating centers, advisory boards, cooperative agreements or other means - in person or virtual." Visiting WHO offices also became prohibited until "further guidance."

FOX News reached out to the CDC and the Department of Health and Human Services for comment, but they did not immediately respond.

In an interview with the Associated Press, a University of Southern California public health expert who collaborates with WHO on work against sexually transmitted infections said the stop-work policy may have major consequences. 

"Stopping communications and meetings with WHO is a big problem," Dr. Jeffrey Klausner, who learned about the memo from someone at the CDC, said. 

Halting collaborations, which Klausner said could "help us protect Americans abroad and at home," could affect how the U.S. monitors certain outbreaks. This includes bird flu outbreaks, which reportedly have affected American livestock, caused human infections, and even raised grocery prices.

"People thought there would be a slow withdrawal. This has really caught everyone with their pants down," Klausner continued.

"China, with a population of 1.4 billion, has 300 percent of the population of the United States, yet contributes nearly 90 percent less to the WHO," the statement said.

During a rally in Downtown Las Vegas on Saturday, Trump said the U.S. pays $500 million a year while China pays $39 million. However, the prospect of rejoining the organization is possible if WHO can "clean it up a bit."

"They offered me at $39 million, they said 'We'll let you back in for $39 million,' they're going to reduce it from [$500 million] to [$39 million], and I turned them down, because it became so popular I didn't know if it would be well received even at [$39 million], but maybe we would consider doing it again, I don't know, they have to clean it up a bit."

FOX News' Alec Schemmel and The Associated Press contributed to this report.