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 staff who work with the WHO must immediately stop their collaborations and “await further guidance.” It also comes as health authorities around the world are monitoring bird flu outbreaks among U.S. livestock.