Bring a Bottle of Disease to Kayneth Stillwind in Forest Song.
The Forsaken, the undead arm of the Horde, have crept into Ashenvale, lurking at our ruins and our barrows. We believe their goal is to spread disease amongst the lands of the night elves. We must confirm this!
The Forsaken have a camp south of here, near the Dor'Danil Barrow Den, where they create poisons and diseases and place them in bottles. Fight your way into that camp and steal a bottle of disease. Return with such a bottle. I will test it and find its purpose.