Civilization IV: City Health Modeling
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. 

For example: too much growth causes overpopulation which leads to an unhealthy city. This unhealthy city leads to less food accumulation thereby resulting in starvation which further compounds the cities unhealthiness.

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