Plague

Plague in Kingdoms and Castles is one of the ways peasants can die and the happiness can drop. It probably happens as transportation becomes easier, and more roads are built. A certain number of people may catch the plague, and to heal them, a hospital needs to be built. It has six doctors who run out and treat the peasants. Since there isn't many doctors, some peasants may still die. The plague spreading itself will cause happiness to drop. People dying of it will also continue to lower your happiness.