How fast would zombies take over the world?

KYLE HILL: How dangerous is a zombie? We all know that we shouldn't let them bite us and that they are impervious to pain, but where does a zombie outbreak rank in terms of public health on a list of pandemics?

So how dangerous would a zombie disease be? Maybe we can look to how we quantify the infectivity of other human diseases? Epidemiologists use a term called the basic reproduction number which is a term quantifying how many cases an infected person might cause in a healthy population.

Now this number, or R₀, can tell you whether a disease will turn into an outbreak or burn itself out. If this is less than 1, the disease is likely to burn out. But if R₀ is greater than 1, you have an epidemic or a pandemic on your hands.

For example, measles is one of the most infectious diseases that we know of, able to cause up to 18 other cases from just one infectious person. Other diseases are much less contagious but still very dangerous. In my last video on how the Walking Dead virus works, I suggested that the virus itself isn't actually killing anybody—everyone already has the virus, so there is no R₀. However, how would epidemiologists of the future looking back on the zombie apocalypse analyse how it first spread?

How quickly would humanity succumb to a Walking Dead-style virus?

Thankfully, we have fellow nerds to help us answer this question. Recently, students from the mathematics departments of the University of Ottawa and Carleton University also in Ottawa tried to model a zombie outbreak to see how fast "slow zombies" would take over the world.

Here's what they came up with. In their model, S stands for susceptible, Z stands for zombie of course, and R stands for removed, or people that have died from the susceptible category or zombies that have been headshotted. Now this is similar to other models epidemiologists use but it gets a little bit more complicated, because people in the susceptible category can die of natural causes, but they can also become zombies through a bite. And zombies can be taken out of the population by being headshotted and going into the removed category. But people in the removed category can reanimate and turn back into zombies. And all of these happen according to some constant rate that you can put into the model afterwards.

According to this model, we would have as little as three days before there are just as many humans as there are zombies, and zombies overtake us shortly thereafter. That's because zombies grow exponentially as we die exponentially.

But there is some hope in the event of a zombie apocalypse. Any option taken against the zombies has to be quick and aggressive. Radical quarantines are likely to fail, so it's going to take "increasing attacks of increasing force" to eradicate the zombie horde. But remember, we only have a number of days before they overrun us. Have you ever known any military or medical organisation to stop something that fast?

Thankfully, we don't know of any disease that can feed back on itself like zombification does. But thinking of zombiism as a real disease does let us think accurately about epidemiology and teach others about public health. Why? Because Science. "Karl? Karl, I wanna teach you about epidemiology Karl!"