I'm not saying Gates is necessarily wrong, but it is awfully convenient that the most important issue for the world just happens to be the one his charity is involved in.
I question whether you can even know what will "save the world". Look at risks to human civilization. What is the impact of malaria on the population versus say, an asteroid crashing into our planet? The latter is more catastrophic to the survival of our species than the former, but the probability of occurrence is much lower.
What if the Internet becomes instrumental to the identification of an asteroid threat with sufficient time to mitigate its effects? Will the Internet have 'saved the world'?
Again, I'm not saying that curing disease isn't important, and I applaud Mr. Gates' efforts even if I may question his motives. But I don't think he can possibly know what will and will not "save the world".