Anomaly UK has gone to the length of learning LaTeX in order to write up his enhanced model of zombie infestation with the proper mathematical notation:
The survival of the human population in the face of a zombie threat as hypothesised here depends on three factors — the ratio of human births to (non zombie-related) deaths, [the ratio] of humans killing zombies to zombies converting humans, and the proportion of corpses which naturally become zombies before decomposing ultimately.
Fortunately, anecdotal evidence suggests that this third factor is very close to zero. That being the case, any influx of zombies can be faced provided we can kill zombies faster than they can directly turn us into zombies.
If this were to change however, and our dead were to start to become undead in serious numbers, we would approach the point where we would not be able to control them. Given a ratio of births to natural deaths of less than two, a kill-to-conversion ratio in human-zombie encounters would have to be well over two to put down newly-risen zombies. The easiest approach to reducing the risk would seem to be to increase lambda (?), that is, to make sure corpses are destroyed before they can become zombies. If 99% of corpses decay or are destroyed safely, then, given a human growth rate of 10% per generation, we only need a 10% advantage in human-zombie conflict to have the upper hand.
A high lambda (?), also, by equation (12), improves the robustness of a society, that is, the ability to overcome a disturbance caused by a sudden influx of zombies or a sudden decrease in able-bodied people able to kill those zombies that naturally appear.