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Illnesses are status effects that villagers may be affected with while performing their duties. Illnesses vary with each village, and can be cured with medicine from Doctors.
Update: Illnesses are currently being revamped. Villagers cannot become ill anymore.
Minor illnesses will decrease your villager's productivity by 50%. For example, Explorers will only be able to explore 10 times per hour while afflicted, and Warriors will only be able to deal half of their normal damage.
Major diseases will render afflicted villagers unable to do their jobs, unless they have Beginner's Boon.
Beginner's Boon is a status effect applied to all villagers that are younger than 2 weeks old. If a villager with Beginner's Boon is infected with a major illness, they will still be able to work, but at half productivity, as if infected with a minor illness.
List of diseases by village
Each village has 5 unique illnesses, with 4 minor illnesses and 1 major illness.
If a villager is sick and visits the clinic, there is a well known bug where they will be cured and they cannot receive any illnesses, even if the pop up says they do. The only way to receive illnesses is from a new villager or a villager that never did a job before. Because of this, it's said that illnesses will be revamped sometime in the future and will be affected to animals.