Name of quest: Mysterious Contagion
When available: After leaving Tanza (?)
Reward: 120,000 G
Description: There's a herbal flower that cures this strange disease. I want you to find it.
Sonnet: Way up north in Yuno, a strange disease has broken out. They're trying to find a remedy that will cure it. This is a quest from a woman named Suda in Yuno. Go there and talk to her directly for details. The reward money is 120,000 G.
As the quest inquires, head to Yuno and talk to Suda (who can be found in the first house to the left). Suda explains to the group that a sickness called the Apolu Fever returned to their village from several years ago and that the medicine to make it requires Gunang Weed. Head to Tanza and talk to Hugo to try and get the medicine to cure Simba. Hugo's home can be found above the rice plants to the east side of Tanza.
Hugo tells Lang that Gunang Weed is really rare and he may be able to find it blooming in The Forest Maze near water. So now head to the northern part of The Forest Maze and look for the river with the camp (which can be found along the northernmost path of The Forest Maze). You can see the plant in the screenshot to the right.
After grabbing the Gunang Weed, head back to Suda (not Hugo!) and give it to her to create the medicine. After she's created it and gave it to Simba, head back to the Guild to get your reward.