Bill Gates, billionaire and co-chair of the Bill and Melinda Gates Foundation, in Washington, D.C., U.S., on Thursday, March 26, 2015.

"Technology is a boom-or-bust business, but it's mostly busts. I've always assumed that 10% of my technology investments will succeed — and succeed wildly. The other 90% I expect to fail," Gates writes. "When I made the transition from my first career at Microsoft to my second career in philanthropy, I didn't think that my success rate would change much. ... Discovering a new vaccine, I figured, would be just as hard as discovering the next tech unicorn. (Vaccines are much harder, it turns out.)" But one type of investment "surprised" Gates "because — unlike investing in a new vaccine or technology, the success rate is very high," he says. "It's what people in the global-health business call 'financing and delivery.'" Gates explains that buying medical supplies and getting them where they're needed can be difficult and dangerous because they often are needed in "remote villages and war zones." He says most people have never heard of the organizations that do this, or their work, but each of the three he's invested in "has been extremely successful.... These organizations are not trivial or expendable. In fact, they are probably the best investments our foundation has ever made."