Diseases are a type of negative Status Effect which the player can receive, in Outward.
Unlike other Effects, Diseases will not go away in time, they require the player to cure the disease in a specific manner. If left untreated, Diseases will progress to a more advanced state with more severe effects.
Diseases do not always cure instantly. Some cures may cause the disease to enter a "receding" phase, indicated by a green arrow on the status effect icon. This means the effect will go away over time and does not require further treatment.
It is also possible to cure some diseases instantly however, such as with Fungal Cleanser.
List of Diseases