Gather Felwood Slime Samples from any of the slimes in Felwood. Then, using the Testing Equipment next to Chemist Fuely in Undercity, see which of your samples are the most corrupt. Bring 5 of the Corrupted Felwood Samples you find to Chemist Fuely.
Felwood is a forest far north of the Barrens, even farther than Ashenvale. There, slimes are found all around pools of water blessed by the touch of Fel. The Scourge decimated the area with attacks of Infernals and other demonic magics--the slimes supposedly crept up through the earth during these attacks.
Gather samples of Felwood slimes and we will use the testing equipment next to me to create a control group for our tests against other samples you gather.