In CIV4 a City's Health stock is a flourishing yet complex condition which acts as a City attribute intended to limit growth. Positive and negative feedback impacts City Health flow and derives from many sources: city's site, buildings, wonders, civics, resources, population, etc.
What drives this flow? When a cities unhealthiness is greater than its health limit, then for each point of population above the health limit consumes 3 food apiece rather than the standard 2 pieces. This further depletes city resources leading to starvation. In addition the city’s health also affects the rate at which it accumulates food.