Some critics say that ending polio has become Bill Gates' "white whale."
Why not just settle for the huge drop in polio cases that we've seen over the past decade and then spend money on other things that kill so many more kids, like diarrhea and malnutrition?
"Polio is special," Gates tells NPR's Robert Siegel on All Things Considered. "Once you get it done, you save $2 billion a year that will be applied to those other activities. There's no better deal economically to getting to zero."
And Gates is putting his money — and his effort — where his passion is.