You need to talk to more imps than that. Many of the sick imps live in the northern section of the village, but not all imps there are sick.....
minor <span class='standouttext'>Spoiler : </span><span class='spoiler'>Take the path north west of the chieftain's house, and talk to the imp in the second house on the right, then report to the chieftain's son.</span> (highlight the text to read)
When you get the quest to find some ingredients to make a cure, make sure you complete that last, since it leads to the end of the act, and you may not be able to do any unfinished quests in the imp village.
