JOHANNESBURG (AP) — Actress Charlize Theron, singer Elton John and Prince Harry are joining researchers, activists and policy makers at a global AIDS conference in South Africa this week to debate ways to better treat and prevent the disease.

The gains the world has made against AIDS are “inadequate and fragile,” U.N. Secretary-General Ban Ki-moon told reporters Monday.

More than half the people around the world infected with HIV, or 20 million people, still don’t have access to treatment, he said.

Countries have pledged to end AIDS by 2030, but activists say more funding is needed, especially to fight tuberculosis, the leading cause of death for people with HIV.

Last week, Prince Harry took a nearly instant HIV test in London as part of his campaign to raise awareness about the virus.