Health officials praised the Nigerian authorities for their transparency in reporting the spread of the virus.

© Arlette Bashizi for The New York Times

The health secretary is quietly taking steps that public health experts fear will reduce immunization rates further.

© Annabelle Gordon/Reuters

The health secretary is quietly taking steps that public health experts fear will reduce immunization rates further.

© Annabelle Gordon/Reuters

Senator Patty Murray, a former chair of the Senate health committee, is forming a group of experts to examine how to insulate the C.D.C. from political interference.

© Tierney L. Cross for The New York Times