Bill Gates has given a lecture in London in which he described the battle to eradicate polio as one of the most difficult tasks the world has ever faced.
The billionaire Microsoft chairman, who launched the Bill and Melinda Gates Foundation, believes that society needs to act quickly, describing the mission as "a test" for the human race.
Delivering this year's Richard Dimbleby lecture, Mr Gates revealed the fight against the disease was the project he dedicated most of his time to.
"Most people in developed countries know of polio as a disease that used to paralyse lots of children. But it's not just a historical curiosity - it still strikes children today," he said. "We are working to wipe the virus off the face of the earth, and we have almost succeeded."
The total number of polio cases worldwide dropped from 650 in 2011 to fewer than 250 last year. There are now only three countries - Pakistan, Afghanistan and Nigeria - which still have polio.
"Stopping these last cases of polio in these last countries is among the most difficult tasks the world has ever assigned itself. But it is also among the most important," Mr Gates warned.
"The best of who we are - our capacity for innovation, our resilience, our sympathy for each other - has gotten us to this threshold. Only disease, poverty and indifference still stand in the way. The fight to eradicate polio is a proving ground, a test. Its outcome will reveal what human beings are capable of, and suggest how ambitious we are about our future."
He firmly believes in the power of vaccination, and said his wife and he were among the leading funders of polio eradication. Universal coverage is one of the key aims of his foundation, which has invested more than 25 million dollars (£15.8 million) in grants. "Vaccines are a miracle tool. They prevent disease from striking, which is better than treating it after the fact. They are also relatively cheap and easy to deliver. However, millions and millions of children don't get them. This is still stunning to me," he said.
Mr Gates concluded his lecture, delivered at the Royal Institution, by calling for the implementation of a "detailed plan" to eradicate polio by 2018. "We cut the child mortality rate by 75% in the past five decades," he said. "We cut the poverty rate by 50% in the past two decades. We eradicated smallpox. These are mind-boggling successes. Adding the end of polio to the list will be one of the great moral and practical achievements of our age."
In giving the annual lecture, aired on BBC One, Mr Gates joined a list of predecessors including President Bill Clinton, outgoing Archbishop of Canterbury Rowan Williams and the Prince of Wales.
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