A new disease-modelling sim mixes classic epidemic maths with factors like bite-to-and kill ratios and discovers full infection of the US always takes about a month. Or [puts on sunglasses] 28 days later.
You can try your hand at the sim here. Cornell University's Alexi Alemi created the model after reading World War Z. It uses a classic epidemic model called SIR and US census population data. The SIR model takes its name from the three factors measured: S, the number of people who are susceptible, I the number of people who are infectious, and R the number of people who recover to become immune.
The twist with zombies of course is that there is no getting better. The only way to 'recover' is for a zombie to be killed. So that means the model depends on one key factor: the ratio between bites and kills, or how effective humans are at taking out zombies. According to Alemi's research (films) a zombie is about 25 per cent more likely to bite a human than a human is to kill it, creating a bite-to-kill ratio of 0.8.
That's then modelled on a map of the US made up of grids populated according to the 2010 US census. Apparently it doesn't really matter where the outbreak starts, or what stats you fiddle with, America falls around a month after the first bite, although remote areas can hold out: Montana and Nevada, for example, remain zombie up to four months later.
My sim's about 70 hours in and... well, I've actually lost the East Coast.
[Zombie simulator lets you plan your own apocalypse - New Scientist]