Your population has a health value. When health is positive, the population grows and provides bonus Unity. If it goes negative, the population will begin to decline.
Your population's health decreases if you emit pollution (air or water). Pollution on your island is global, so moving smoke stacks farther from your settlement won’t help. It is a good idea to keep your eye on how much pollution you emit.
Your population can suffer from diseases. Some diseases come and go on their own, and it’s not possible to completely prevent them. However, providing healthcare can significantly reduce their negative impact.
Diseases can also occur due to poor conditions in your settlement. For example, a lack of clean water or too much trash in the settlement can lead to disease. Pollution does not cause diseases directly, but instead applies a direct health penalty.