How this calculator works
The model is a two-group version of a compartmental epidemic model. Each month, every zombie infects a fixed number of humans, limited by the number of humans left. Humans kill zombies in proportion to their own population, and a fixed share of the zombies decays. The infected humans leave the living group and join the zombies in the same step. When the infection term exceeds the sum of kills and decay, the zombie count grows; when it is smaller, the outbreak fades. Human growth or decline runs in parallel, so a growing population can outpace a slow outbreak while a shrinking one falls faster.
Formula
Each monthly step: humans infected = the smaller of the human population and zombies x bite rate. zombies killed = the smaller of the zombie population and humans x kill rate / 100. zombies decayed = zombies x decay share. Humans next month = humans x (1 + annual growth / 12) - humans infected. Zombies next month = zombies + humans infected - zombies killed - zombies decayed.
Worked example
A thousand humans and a hundred zombies, with a bite rate of 0.1 humans per zombie per month and no defence or decay, have ten infections in the first month. Humans fall to 990 and zombies rise to 110, and the following month brings eleven infections. With a defence rate of 100 zombies killed per 100 humans per month, the first month kills all hundred zombies and the humans finish unchanged at a thousand. With both rates at zero the populations never move.
Advertisement
Frequently asked questions
Is this based on real epidemiology?
It uses the same bookkeeping as a simple compartmental model, but the parameters are invented and the population is treated as one well-mixed group. Real outbreaks depend on contact networks, geography and behaviour.
Why do the zombie numbers sometimes collapse?
Kills and decay both remove zombies, and both scale with the zombie count. If their sum exceeds the number of new infections, the outbreak shrinks toward zero rather than growing.
What do the rates mean in everyday terms?
The bite rate is how many people one zombie infects each month, the kill rate is how many zombies one hundred humans remove each month, and decay is the share that simply wears out. Adjust them to match the story you want to explore.
Can the human population recover?
If the outbreak is contained, positive human growth can rebuild the population over many months. The model has no immunity or cure, so recovered humans simply remain human.