Malaria
Malaria causes fever, tiredness, vomiting, and headaches. In severe cases it can cause yellow skin, seizures, coma, or death. Like other mosquito-borne diseases, the key to prevention lies in controlling mosquitoes (see Mosquito for more on how people in the Fifth World do that).
While several plants offer help with malaria, overuse in ancient times helped the disease develop resistance to all of them to one degree or another. Four hundred years with a much smaller and less dense human population to spread amongst has softened that resistance, but all healers know that treating malaria requires a combination of quinine, sweet wormwood, and other medicines used in combination.