The Trump administration has been negotiating agreements with dozens of individual countries. Public health experts say a global agreement would be far more effective.

© Arlette Bashizi for The New York Times

The Trump administration has been negotiating agreements with dozens of individual countries. Public health experts say a global agreement would be far more effective.

© Arlette Bashizi for The New York Times

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

© Arlette Bashizi for The New York Times