World of Warcraft is not the real world, and its zombie plague was clearly very different from any real diseases. Yet the infection seems to have been carefully designed to mirror at least some aspects of genuine epidemics, according to scientists.

While the “corrupted blood” plague that accidentally spread through the game a few years ago shared few characteristics with communicable diseases such as measles, flu or smallpox, this week’s zombie epidemic was a little more true-to-life.

The disease was highly contagious, but, like real pathogens, it did not inevitably infect every player who had contact with a case.

“This was quite cleverly designed to be similar to real diseases, in that not everybody gets it, but the probability of infection goes up as you encounter more cases,” said Professor Neil Ferguson, director of the MRC Centre for Outbreak Analysis and Modelling at Imperial College, London.

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“Meet one zombie, and you had a good chance of avoiding infection. But if you encounter a group of five to ten, you’re almost certain to be infected.”

That contrasted with the earlier outbreak, which infected everybody who came into contact with it. Such 100 per cent transmission rates do not exist in nature.

No virus or bacteria is yet known that can cause infected people to explode, as infected zombies could, but this aspect of the disease does have some parallels in nature.

An exploding zombie would spread infectious material over a wide area, passing the contagion on to others. Many human pathogens do this in rather less spectacular fashion, causing symptoms such a sneezing, vomiting or diarrhoea that make hosts more likely to transmit germs to others.

The most important factor for the spread of any communicable disease is its reproduction number, or “R number”, which is the number of people to whom an infected person will typically transmit the pathogen in a population with no immunity. If the R number is greater than one, the epidemic will spread, while if it is lower than one, it will decline.

The most contagious diseases, such as measles, can have R numbers that reach 15 during the height of epidemics, though lower values are more common. The R number of smallpox is generally about 5, though it can reach as high as 10, and influenza typically has an R number of about 2.

As Blizzard would not release details of the number of infections over time, it was impossible to calculate how the zombie plague’s R number had altered over time. Professor Ferguson said a value of about 5 seemed likely.

This would have been expected to grow had Blizzard not intervened. “It would eventually have gone up to 10 or even 20, and you’d have seen 90 to 95 per cent of all players becoming infected,” Professor Ferguson said.

The dynamics of the plague were not an exact match for the real world, but they were a much better fit than those of corrupted blood. “That seems to have had a reproduction number into the hundreds,” he said.

Some epidemiologists, such as Nina Feffernan, of Tufts University, have suggested using online games to study the spread of diseases, but Professor Ferguson is sceptical because they cannot properly mimic genuine behaviour.

In the early stages of the zombie plague, for example, many players deliberately set out to become infected, to gain zombie powers. Characters who die can also be regenerated: there is not quite the same incentive to avoid like the plague.

Online games such as World of Warcraft could potentially be set up to help scientists to study epidemics, but their utility would always be limited by their primary purpose of entertainment, Professor Ferguson said.

“It might be possible to design something that would give some insight into the behavioural dynamics you’d see in an epidemic. But you have to remember it’s a fantasy universe we’re talking about. People can fly places. You come back to life when you die. It’s difficult to map that on to reality.”

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