Problem: AIDS and hepatitis are spread by reused needles.

Solution: A syringe that can't be shared.

Each year 23 million people in the developing world contract hepatitis and 260,000 get HIV from reused syringes, according to the World Health Organization. Twenty-two years ago Briton Marc Koska, 45, saw a solution: a syringe that self-destructs after one use. With seed capital from friends and family, he set about absorbing everything he could about how syringes are used - and misused - around the world.

He reached one important conclusion: "The world didn't need any more factories," says Koska. "If I could go to a factory and get them to convert, I could stop them from making bad syringes and help them make the good." Koska had to design a syringe that could be made with existing equipment and persuade makers to license his design. It was 17 years before his company, Star Syringe, sold its first single-use syringe in 2001.

Today Star Syringe has 16 licensees making and selling 350 million of its K1 devices in 25 developing countries. The K1, says Koska, has helped save more than two million lives. Thanks in part to a nationwide implementation of Star Syringe's needles, for instance, Uganda has cut AIDS infection rates in half since 2003, dropping from one of Africa's highest infection rates to among the lowest. --John Simons