Max Brooks' epic novel World War Z presents a painstaking and frighteningly credible account of the way that a zombie plague could spread worldwide from a single initial case. The Marc Forster film, however, changes a few of the rules of contagion - and with it the likely outcome of the disease. We assembled a few facts from the film and presented them to Dr Bradley Forssman, Public Health Physician, and epidemiologist George Truman for an assessment of what's realistic and what's not for this important public health issue. Forssman talked us through the plot, while Truman very kindly modelled the infection rate. There are some spoilers for the film here, so tread with caution if you're yet to see it...
|If we accept that this infection exists and has the characteristics that they claim, it is likely that the infection would spread through a population very rapidly|
The infection's spread by bite / bloodstream. You can't ingest infected blood and become infected, nor does it seem to be airborne, or present in water or food or on surfaces. Once bitten, you turn zombie in 12 seconds and live to bite others and spread the infection further. These zombies can run, jump and swarm into towers that allow them to scale walls.
"If we accept that this infection exists and has the characteristics that they claim - i.e. an incubation period of 12 seconds, high intensity of exposure, and 100% chance of transmission with exposure through bite/bloodstream - it is likely that the infection would spread through a population very rapidly. How rapidly would depend on how quickly isolation / quarantine occurs (if at all) and how crowded the particular population is (that is, the chance of contact with an infected person would be high in Shanghai, for example, but low in the middle of the Australian outback). So that part makes some sense."
At the start of the film, there are reports of riots / unrest on TV but Our Hero and his family are calmly preparing a day trip, so presumably what's happening doesn't seem widespread or serious in Philadelphia. Within 24 hours the whole eastern seaboard of the US has been swept by zombies and the whole US is basically gone. We're told that the first report mentioning the word "zombie" came from South Korea (specifically a US military base there) ten days before the story begins. Patient Zero is rumoured to have come from Taiwan before that.
"After much serious discussion with my staff here about this issue, we have come up with the following numbers to demonstrate how fast the infection would spread through a non-immune population, starting from one infected person. These assume that each infected person would pass on the infection to 20 other people per day, and that the population is infinite."
"So given the population of the US is 313 million, according to this model, the whole population would be infected within a week of the first zombie arriving on its shores.
"In reality, the "number infected per day" would decrease as more people became infected, as it would become more difficult for infected people to find non-infected people to infect. So I have also worked out a scenario where the number infected per day reduces by 1 person per day after the second day."
"So not really much of a change there. Of course, these are very simple models that do not take into account the behaviour of non-infected people (i.e. running away and hiding), or for how long infected zombies live. I suspect that adding those variables into the equation would not make much of a difference given how large these numbers are."
|I'm not sure how the outbreak would have spread from Taiwan to South Korea without alerting authroities around the world... But that would have been a very short and dull movie.|
There is a remark that the plague has spread through airline hubs.
"What seems ridiculous at first glance is the spread around the world through airline hubs. Given the 12 second incubation period, an already infected person would have to board the plane in his/her zombie state, presumably biting people on the way and infecting everyone on board and in the airport. Surely this would only happen once before airlines/governments would shut down operations and stop international travel. Even if the aeroplane took off with a load of infected people on board, they would eventually bite and infect the pilots. Unless the zombies maintain high level cortical functioning in addition to their insatiable desire to bite others, I'd imagine the plane loaded with infected people would go crashing into the nearest mountain/ocean/major population centre fairly quickly after take-off.
"I'm not sure how the outbreak would have spread from Taiwan to South Korea without alerting authroities around the world, and travel stopping at that stage. But that would have been a very short and dull movie."
The zombies live to bite others and spread the disease further. The zombies ignore anyone suffering from other serious / terminal diseases and literally walk / run right past them, because we're told that the disease doesn't want competition as it attempts to spread itself. Our Hero, at one point, infects himself with typhoid or something (it's serious but curable, anyway) and essentially becomes invisible to the undead.
"All I can say about that is, what a great plot device that allows Our Hero to escape certain demise. This sort of behaviour does happen at the microbiological level, where some viruses and bacteria won't infect cells that are already damaged, but it is unlikely to occur at the macro level. How does the zombie know that someone has cancer, for example? This is a particularly ridiculous and unrealistic scenario and convenient for the plot to move forward. I much prefer the method used in The Walking Dead: covering yourself with dead zombie mush and walk like a zombie through the zombie crowd, hoping like hell it doesn't rain."