Leper Gnomes are diseased, corrupted versions of regular Gnomes and villains in the online game, World Of Warcraft and are hostile to almost all species.
When Gnomeregan was invaded by a horde of troggs unearthed from the Uldaman excavation, High Tinker Gelbin Mekkatorque ordered the opening the pressure valves of their giant, grinding machines and released toxic radiation (via an exploded radiation bomb) throughout the city. Many more gnomes were killed than troggs when Gnomeregan was flooded with radiation. Forced to flee to the safety of the dwarven capital of Ironforge, Mekkatorque had to deal with the guilt of his actions that day, actions brought on by his chief advisor's lust for power.
Even worse, though, many gnomes still within Gnomeregan's halls became horribly mutated and insane, changing into what are now known as leper gnomes. Leper gnomes vaguely resemble normal gnomes. Emaciated and sickly, their skin is bloated and green with gangrene, while their eyes glow with a color and light similar to that of the radioactive pools in their once beautiful city. Even worse than their physical mutation, though, is the perverse madness caused by the poisoning that twisted their bodies. While individuals among them may seem saner than others, all are irrevocably twisted. Leper gnomes claim Gnomeregan as their own, and, led by Mekgineer Thermaplugg, defend its halls with horrible constructs and their own prowess. They continue their battle against the troggs, who are more dangerous now due to the radioactive goo that clings to their rock-like skin. Perhaps the worst enemies in their minds, are true gnomes, whom they blame for their current existence. They regard High Tinker Mekkatorque as a demon, and his followers not much better. They regard all other creatures as intruders and quickly put them down. Recently, however, some adventurers have seen leper gnomes in company with Dark Iron dwarves. Many theorize that Thermaplugg is attempting an alliance between the two underground races, in an attempt to strengthen his own forces and finally defeat the troggs.