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Experts said the sudden stoppage was a surprise and would set back work on investigating and trying to stop outbreaks of and in Africa, as well as brewing global threats. It also comes as health authorities around the world are monitoring among U.S. livestock.

The Associated Press viewed a copy of Nkengasong's memo, which said 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.� It also says CDC staff are not allowed to visit WHO offices.

President Donald Trump last week issued to begin the process of , but that did not take immediate effect. Leaving WHO requires the approval of Congress and that the U.S. meets its financial obligations for the current fiscal year. The U.S. also must provide a one-year notice.

His administration also told federal health agencies with the public through at least the end of the month.

“Stopping communications and meetings with WHO is a big problem," said Dr. Jeffrey Klausner, a University of Southern California public health expert who collaborates with WHO on work against sexually transmitted infections.

“People thought there would be a slow withdrawal. This has really caught everyone with their pants down,� said Klausner, who said he learned of it from someone at CDC.

“Talking to WHO is a two-way street,� he added, noting that the two agencies benefit from each other's expertise. The collaboration allows the U.S. to learn about new tests, new treatments and emerging outbreaks — information “which can help us protect Americans abroad and at home,� Klausner said.

The CDC details nearly 30 people to WHO and sends many millions of dollars to it through cooperative agreements. The U.S. agency also has some of the world's leading experts in infectious diseases and public health threats, and the two agencies' staffers are in daily contact about health dangers and how to stop them.

The collaboration halt isn’t the only global health effect of Trump’s executive orders. Last week, the president froze spending on another critical program, PEPFAR or the President’s Emergency Plan for AIDS Relief.

The anti-HIV program is credited with saving 25 million lives, including those of 5.5 million children, since it was started by Republican President George W. Bush. It was included in a Trump administration slated to last at least three months.

PEPFAR provides HIV medication to more than 20 million people “and stopping its funding essential stops their HIV treatment,� International AIDS Society President Beatriz Grinsztejn said in a statement. "If that happens, people are going to die and HIV will resurge."

A U.S. health official confirmed that the CDC was stopping its work with WHO. The person was not authorized to talk about the memo and spoke on condition of anonymity.

A WHO spokesperson referred questions about the withdrawal to U.S. officials.

Officials at the U.S. Department of Health and Human Services didn't immediately respond to an emailed request for comment. And CDC officials didn't respond to the AP's request to speak with Nkengasong about the memo.

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AP Medical Writer Lauran Neergaard contributed to this report.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.