How did Miles O'Brien loose his arm?

Miles O'Brien lost his arm in the "Star Trek: Deep Space Nine" episode titled "Armageddon Game." During a mission on a planet, he and Chief O'Brien were exposed to a deadly viral weapon, and in the process of trying to contain it, O'Brien's arm was severed. He later received a prosthetic arm, which became a part of his character's evolution throughout the series.