Bipartisan support 2 decades on

PEPFAR is up for its fourth authorization from Congress this year. It is expected to move forward, since the program has long been seen as a bipartisan success story.

"It enjoyed strong bipartisan support historically and that hasn't changed — despite changes in administration, despite changes in Congress, it's still received robust support on the Hill," Moss said.

On a visit to South Africa on Thursday, five U.S. senators vowed to renew PEPFAR, with Sen. Lindsey Graham calling it "the right thing to do" at a press conference.

Every five years, Congress signals its priorities when reauthorizing PEPFAR — for example, asking the program to focus on children orphaned and made vulnerable by HIV/AIDS or providing nutritious food to those affected by HIV.

Some of those initiatives have been misses. The U.S. spent $1.4 billion promoting abstinence to prevent HIV, which proved to be a failure. Critics said that program was an attempt to force certain values on other nations – legislation meant to appease conservative lawmakers that was later phased out.

"Is there room for improvement? Yes, we continue to learn lessons," Nkengasong said of missteps.

Yet, he said, "PEPFAR is, in my view, the greatest act of solidarity that humanity has ever created in solving an infectious disease challenge like HIV/AIDS."

During their visit to South Africa last week, the five U.S. senators met with a young woman who was diagnosed with HIV — and went on treatment — years ago, Nkengasong said. She's now married, with two young daughters who are HIV-negative. Stories like hers show the importance of reauthorizing PEPFAR, he said.

"We have millions of people who are in treatment now, and if we take our foot off the pedal, then we will begin to see the virus reappear" among those in treatment — and spread to others, Nkengasong said.