As the reality sets in that the United States is drastically diminishing its foreign assistance to developing countries, an urgent conversation is starting among governments, philanthropists, and global health and development organizations.

It is centered on one crucial question: Who will fill this gap?

Last year, the United States contributed about $12 billion to global health, money that has funded treatment of H.I.V. and prevention of new infections; children’s vaccines against polio, measles and pneumonia; clean water for refugees; and tests and medications for malaria.