4.25 billion people, more than the entirety of Asia, have suddenly ceased to exist or

are just no longer people. And scenarios like this are nothing new, there have been hundreds

and hundreds of movies and videogames that cover similar events, but could a pandemic

like what is depicted in The Last of Us...actually happen?

You only have to go back 95 years to 1918, at the end of World War 1, to witness one

of the most deadly and disastrous pandemics in human history: The Spanish Flu.

This variety of avian influenza killed an estimated 50 to 100 million people, more than

3 times the amount who died in the actual war, and with a worldwide population at the

time of 1.85 billion, it wiped out 3-5% of the entire planet; and that was only in one

year.

But the flu is a virus that actually exists, and in the work of fiction The Last of Us

we are dealing with an organism, a fungus...that also...actually exists.

Cordyceps are fungal parasites that infect and take over insects. The most well known

being ophiocordyceps unilateralis that infects ants, turning them into zombies. A spore from

the fungus enters the ant's body, taking over its brain and controlling its movements. Not

too long after, the ant finds a perfect spot to die, the fruiting body of the fungus grows

out of the ant's head, and a slew of spores are released onto the unsuspecting ant population.

It has been known to decimate entire colonies, but luckily it only infects insects, and it's

not like diseases can move from one species to another...well....

An estimated 60% of all modern diseases came from animals, specifically ones that we live

in close contact with or have domesticated. From cattle grew measles, tuberculosis and

smallpox. From pigs and birds we have the flu. From chimpanzees we have AIDS. All of

these started out as infections that couldn't be spread to humans, but after years and years

of these microbes being around us, they adapted to our biology.

But we don't live in direct contact with ants, and most people don't eat ants, so that's

good...but other animals do, and then other animals eat those animals, and at some point

on this parasitic food chain lie humans. Considering how diseases mutated and progressed from domesticated

animals to us, it isn't too far fetched to think that a fungal infection like this, whose

only purpose is to survive long enough to spread, could adapt and evolve to lay claim

to the top of that food chain.

A great example of a microbe's ingenuity is in Jared Diamond's amazing book 'Guns, Germs

and Steel'. Jared talks about the bacterial disease typhus and how it was transmitted

to people by going from rat fleas to rats to humans. But after a while, typhus figured

out a much easier way to infect us: by cutting out the unnecessary carriers and going directly

from human body lice to humans.

If a fungal parasite like the Cordycep could infect humans like it does in The Last of

Us, how long would it take? One of the biggest differences between the Spanish Flu in 1918

and a modern day pandemic is how connected we are. What would have taken two weeks to

go from New York to Germany by boat, can be done in 8hrs on a plane. Using the Global

Epidemic and Mobility software and some parameters we know from the game, we can create a model

and simulate what the transmission and spread would look like.

In about 60 days, the fungal parasite would have infected the majority of the planet.

And you know things are bad when not even Madagascar is safe.

But that isn't to say that the governments of the world don't have plans in place. Most

governments - and the United States in particular - have strategies for biological threats,

counterterrorism, and the one that applies to this situation most, The National Strategy

for Pandemic Influenza. In it their high estimate for fatality domestically is 2 million, or

less than 1% the population of the US. This incredibly detailed 233 page document really

drives home one main point, and that is "sustain infrastructure and mitigate impact to the

economy and the functioning of society".

A virus or disease or parasite doesn't affect power lines, cars, the internet...it affects

the people who make those things work. When 60% of the entire population, over 4 billion

people, just stop being - when the doctors, the teachers, the mechanics, the mail men

vanish, and what we know as society crumbles...all that's left is...the last of us.

And as always, thanks for watching.