NEW YORK (AP) U.S. public health officials have been told to stop working with the World Health Organization, effective immediately.

A U.S. Centers for Disease Control and Prevention official, John Nkengasong, sent a memo to senior leaders at the agency on Sunday night telling them that all agency staff who work with the WHO must immediately stop their collaborations and “await further guidance.”

Experts said the sudden stoppage was a surprise and would set back work on investigating and trying to stop outbreaks of Marburg virus and mpox in Africa, as well as brewing threats from around the world. It also comes as health authorities around the world are monitoring bird flu outbreaks 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 Trump last week issued an executive order to begin the process of withdrawing the U.S. from WHO, 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 to stop most communications 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 WHO and U.S. health officials benefit from each other’s expertise. The collaboration allows the U.S. to learn about new tests and treatments as well as about emerging outbreaks information “which can help us protect Americans abroad and at home.”

The CDC order isn’t the only global health effect of Trump’s executive orders. Last week, he 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 but was included in a freeze on foreign aid spending 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. If that happens, people are going to die and HIV will resurge,” International AIDS Society President Beatriz Grinsztejn said in a statement.

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.

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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.