Lurkers are strange beings who find their home in mucky water, swamps, sewers and anything of the like.
They were created when a Water mage tried to make a water elemental, but miscalculated. The spell affected another student, turning them into a shambling mess of slime and a sinister form. The slime seemed to alter anyone who the former student touched, slowly infecting others that the Lurker touched.
The Lurker disease is easily spread through the slime they naturally produce with excess amounts of water. The infection spreads slowly through the body. First, the infected complain of irritated skin, which over the course of a week or two, becomes a pale green color. Second, they start scratching uncontrollably, which speeds up the infection if they continue. Third, most of their features become obscured by the slime that their skin secretes. Lastly, any idea that the person is still there is gone. The infection has swallowed them, tainting their very Souls, destroying memories and everything other than survival instincts.
Despite these horrifying facts, they are actually easily dealt with. Since they're naturally slow, simply dispatch them from a distance. Just avoid any suspicious green slime you see.