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Crop fields, pastures, and orchards can develop pest infestations. Nearby fields that are growing the same crop or raising the same animal may become infected once a pest infestation breaks out.
When infestations do occur, you can harvest crops early, or move animals to other pastures. After the infestation has passed, changing the crop or animal that is grown in a field will reduce the chance of a re-occurrence.
Infestations do not happen when you turn disasters off at the start of the game.