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How’s The Science Of World War Z?
We ask doctors to assess the realism of the zombie plague...

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."

Days Cases

0 1
1 21
2 441
3 9,261
4 194,481
5 4,084,101
6 85,766,121
7 1,801,088,541
8 37,822,859,361
9 794,280,046,581
10 16,679,880,978,201
11 350,277,500,542,221
12 7,355,827,511,386,640
13 154,472,377,739,119,000
14 3,243,919,932,521,510,000
15 68,122,318,582,951,700,000

"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."

Days Cases

0 1
1 21
2 420
3 7,980
4 143,640
5 2,441,880
6 39,070,080
7 586,051,200
8 8,204,716,800
9 106,661,318,400
10 1,279,935,820,800
11 14,079,294,028,800
12 140,792,940,288,000
13 1,267,136,462,592,000
14 10,137,091,700,736,000
15 70,959,641,905,152,000

"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."

World War Z

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."

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Your Comments

Posted on Monday July 15, 2013, 15:29 by Gouken
Speaking of science, the zombie/ant analogy doesn't work because the ants can form those pyramids because their mass is.. well.. hum.. less! 3 miligrams is the avg ant, against let's say 80 Kg for a human. And an infection can only spread quickly via air hubs or other forms of mass transit if the incubation period is long enough for the infected people mingle amongst the uninfected. If you turned into a raving zombie in 12 seconds I don't think many planes would leave the ground under those circumstances. I love the book (and the Survival Guide) I'm trying hard not to hate the film, as I haven't even seen it yet, but I'm failing. To try to enjoy it I'll have to pretend it's an unrelated Zombie film Read More

2 terrible
Posted on Tuesday July 9, 2013, 12:47 by purplegnome
watched this last week loved the book, what a waste of time this film was hopefully the asylum and sci fi channel will do a cheap knock off that will be closer to the source material instead of the 150 million pile of crap that pitt and marc foster gave us. Read More

3 Zombies Grow A Brain, Brad Grows An Expression.
Posted on Monday July 8, 2013, 13:30 by BenTramer
If there's to be a sequel, then the zombies will have to catch-on to the trick of humans deliberately infecting themselves with other viruses or diseases as a ploy to avoid being attacked. Another thing that needs to change is Brad Pitt's odd blank expression all the way through the movie, even when he's cornered by a terrifying zombie that's glaring at him from behind glass - no reaction whatsoever. Read More

4 a little too convenient
Posted on Sunday June 30, 2013, 13:41 by findogask
Yes have to agree kinda went along for the ride, but did think " this is nice and convenient " way to wrap everything up neatly, just in case we don't get a sequel ( and I don't think they will) Read More

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