Plague can be caught by eating rotten food, standing next to infected corpses or transmitted by NPC bandits. Plague can be spread from player to player through coughing, sneezing, and general close contact.
End of map is also bordered with plague and if player come to close they might get more and more infected. Plague has few stages and most of them can be cured by using player or monk trader items. All infection stages vary from Headache (reduces XP gained) to dysentery and eventually death.