Microsoft founder Bill Gates said the battle to eradicate polio was one of the toughest the world has faced, but said it could be conquered by 2018.Delivering the annual Richard Dimbleby lecture in London on Tuesday, Gates, the United States’ richest man, said ridding the world of polio would be "one of the great moral and practical achievements of our age”.The 57-year-old businessman turned philanthropist, who is putting his resources into the fight, said that though polio was still endemic in Afghanistan, Pakistan and Nigeria, vaccination campaigns could eliminate it within six years."We are working to wipe the virus off the face of the earth, and we have almost succeeded: There are only three countries in the world where the virus is still being transmitted. Fewer than 250 children were paralysed last year,” he said."Stopping these last cases of polio in these last countries, however, is among the most difficult tasks the world has ever assigned itself."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 can be about our future.”Polio -- which afflicts mainly the under-fives causing death, paralysis and crippled limbs -- travels easily across borders and is transmitted via the fecal matter of victims. Agencies