Zombie films often end with a miracle cure or the arrival of the army (or the protagonists and the world facing uncertain doom). But what would happen if a new disease started converting humans into a zombie-like state for real, and how might we actually stop it? Answering this kind of question isn’t just a fun exercise – it can tell us a lot about how a genuine highly infectious condition might spread.
Scientists who study disease – epidemiologists – need to answer questions such as how a disease spreads, how fast it is likely to move through a population and, most importantly, how best to stop the outbreak and prevent future events.
Imagine if a new “zombieism” disease broke out that spread through the commonly portrayed method of infected saliva that enters a person’s bloodstream after being bitten by a zombie. This kind of direct contact spread would normally be quite an inefficient method for disease transmission compared to airborne infections, which can be spread much more easily. But zombieism is interesting because, like rabies, its symptoms can include anxiety, agitation, paranoia and terror that alter the behaviour of the infected individual. So zombies will actively try to bite or eat their victims, thereby spreading the disease faster.
Finding the source
The original source of the zombie outbreak is rarely a concern in horror films, whereas epidemiologists spend a lot of time using analysis and models to track back to the first case(s) of disease. For example, a biological agent released into the air or contaminated food consumed at a large social event might cause a large number of infected individuals at one time. A laboratory accident, on the other hand, might initially affect a reasonably small, possibly contained number of individuals, in a small area. These parameters, as well as the mode of transmission, will affect the disease’s spread.
Armed with an understanding of the transmission method, we can consider how fast the disease could move through a population. For any epidemic, epidemiologists will try to calculate a basic reproduction ratio that describes the average number of additional cases an infected individual will generate. This number (often referred to as R₀) describes how severe an outbreak is. One with an R₀ of less than 1 will eventually die out, but greater than 1 will spread through a population.
Our zombies do not expire naturally and require the removal of their head or destruction of their brain for permanent death. So the main consideration in a zombie epidemiology model is how many people a zombie can bite before it either runs out of victims or is destroyed. This is a function of many other variables including population density and people’s ability to destroy zombies. The zombies might be the slow shambling monsters of tradition, or perhaps the faster, more deadly infected humans portrayed in the film 28 Days Later.