The population may get sick naturally, or as the town grows as a result of traders or nomads coming to the town. The population can suffer from diseases such as influenza, plague and everything in-between. Some diseases spread quickly, some linger, some have high fatality rates, some low. The diseases will spread from citizen to citizen based on health factors and proximity. Physicians and Herbalists can help keep the population healthy and cure the diseases in some cases.
Taking in nomads significantly raises the risk of an outbreak of a disease.
Although foresters can plant new trees, the cures for many diseases can only be found by Herbalists in forests that have a large number of mature trees, since herbs do not spawn under tress until they are fully grown.