In my third annual letter, I make the case against cutting foreign aid for the poorest, even in the current budget environment. Investing in aid works. It has had a huge impact on the lives of poor people, and it also helps people in donor countries by promoting stable and prosperous societies across the globe. Whether you think it's an issue of morality or enlightened self-interest, aid spending is uniquely effective spending, and I wanted to give some examples.

The most important example is vaccines. Vaccines are the best investment in health. Just a few doses of a vaccine protects children from debilitating and deadly diseases for a lifetime. And most of them are very inexpensive. The measles vaccine costs 18 cents. The polio vaccine costs 13 cents. That's a small price to pay to save a life.

In the case of polio, we've increased coverage of the vaccine so that we're on the threshold of eradicating the disease altogether. The number of cases has gone down more than 99 percent in the past 20 years, and I think we can bring it all the way down to zero with the continued support of donor countries and a concerted push in polio-affected countries.

Running across all the issues I touch on in the letter--foreign aid, vaccines, polio, and more--is the theme of leadership. I talk about British Prime Minister David Cameron, who has been a bold leader on maintaining aid spending. I also talk about Jim Grant, the former head of UNICEF, who led a massive campaign in the 1980s to get vaccination rates up from 20 percent to almost 80 percent. In so many of the areas where the foundation works, the difference between success and stagnation is powerful leadership.

I hope when people read my letter, they see how optimistic I am about the future. Even though I'm impatient about some things, that's only because I see so many opportunities to make progress very quickly.