Freshly back from Ethiopia and Zambia, philanthropist Bill Gates Wednesday shared with a Stanford audience his optimism and frustrations in addressing health problems and poverty in the world's poorest countries.

The Microsoft founder, who now works full-time in philanthropy, showed a detailed command of nagging issues in health and farming and a data-driven approach to attacking them.

Though progress has been made against child deaths, 20 percent of kids in poor countries -- down from over 30 percent -- still die by age 5 from readily preventable and curable conditions like malaria and diarrhea, Gates said.

Regarding overall poverty, just over 20 percent of people in the developing world exist on less than $1.25 a day, down from over 50 percent in 1981, largely due to progress in China.

Much of the progress is due to better seeds, improved nutrition and economic growth in Asia, as well as vaccines -- and the eradication of smallpox -- that have "reduced disease very dramatically," Gates said.

But the green revolution "didn't happen in Africa -- so Africa today has the lowest agricultural productivity by far of any place in the world, and that's one thing we have to change to bring down the hunger number," he said.

"What makes me impatient is that normal market signals don't cause us to prioritize this work -- substantially more is spent on drugs to eliminate baldness than malaria, which is killing over 800,000 children a year.

"So we need a little philanthropy for people whose voice in the marketplace is unnaturally weak," he said.

With an endowment of more than $37 billion, the Bill & Melinda Gates Foundation is the world's largest, focusing on global health and poverty and, in the United States, expanding educational opportunity and access to information technology.

Gates summarized three of his projects in Africa -- a $30 million investment in a meningitis vaccine that so far has reached 50 million people; a campaign for male circumcision to reduce the spread of HIV until an AIDS vaccine can be developed; and a triple-layer bag to protect harvested crops against weevil invasion.

Solutions require "innovations" that combine deep, sophisticated science -- as in the development of the meningitis vaccine -- with simple but critical measures such as keeping it refrigerated even in remote areas so it remains effective, he said.

"I'd say we should be quite hopeful about progress if we can get innovators to care about these projects and rich countries to stay interested, which is in jeopardy right now because of foreign aid budgets," Gates said.

Gates, who dropped out of Harvard University in the 1970s to launch Microsoft, said college fueled his fascination with microprocessors but failed to give him a larger view of the world.

"I definitely got through school without having any sense of how the poorest in the world live -- what it means to have no infrastructure, a government that doesn't work.

"It was like a come-uppance to learn about all these things later.

"It's great to pursue curiosity in school, but you also have to have a broad view of things and I definitely missed that," he said.

Gates handed out free copies of the book "Getting Better: Why Global Development is Succeeding -- and how We Can Improve the World Even More," by World Bank senior economist Charles Kenny.

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