The donation comes in the form of a promissory note, not as cash, which the Gates Foundation said "gives the Global Fund the flexibility and authority to distribute funds efficiently based on immediate needs."

"By supporting the Global Fund, we can help to change the fortunes of the poorest countries in the world," Gates said in a statement. "I can't think of more important work."

At a news conference at the World Economic Forum in Davos, Gates said the funds could be used immediately to "saves lives, whether it is bed nets (to protect against mosquitoes carrying malaria) or TB (tuberculosis) treatment, those are two diseases that don't get perhaps the visibility of the work done in HIV but they are every bit as important."

The investment comes on top of $650 million the Gates Foundation has already contributed since the Global Fund was launched 10 years ago.