Editor’s note: This article is part of an ongoing WPR series inviting authors to identify the biggest priority—whether a threat, risk, opportunity or challenge—facing the international order and U.S. foreign policy today.

The continued impasse in Congress over appropriating funds to combat the Zika virus in the United States perfectly illustrates the challenges that the next American president will face in addressing global health. There is a generalized sense that something needs to be done, but widespread disagreement over who should do what—and who should pay for it. ...