Snakes kill or cripple 500,000 people a year, but antivenins are costly and rare in poor countries. Now scientists are testing injectable nanoparticles that neutralize venom.

In wealthy countries, snakes are an abiding threat to an unlucky few, among them hikers, ranch hands, soldiers, zookeepers and reptile collectors.

In the tropics of Africa, Asia and Latin America, however, they are a major cause of death and disability in rural areas: more than 2 million people are bitten each year. About 100,000 of them die, and another 400,000 are left with serious disabilities, including amputations or nerve damage so extensive that a leg or hand is permanently useless.