Is zombie transmission transmission frequency or density dependent?

For reasons that I cannot explain, visitors have started to stumble upon my blog by googling the question, “is zombie infection frequency or density dependent?” Maybe there’s a really awesome educator out there using that example in class. Or maybe the zombie apocalypse has started and people are secretly beginning to plan for the end. Either way, this is a neat question that I’m willing to speculate about!

First, we need to decide what kinds of zombies we are talking about. Let’s assume we’re looking at World War Z type zombies, where infection is transmitted via bites/saliva/fluid transfer. Let’s say that the zombies are highly mobile and thus the human and zombie populations are well-mixed. Also, let’s assume that zombies don’t really have a contact structure, like humans do, because they’ve lost any kind of social system that they had a humans.

Given those assumptions, I would expect disease-relevant contacts to increase with host density. So, if I had to pick between density dependent and frequency dependent transmission, I’d expect density dependent transmission. But don’t forget that there are nonlinear contact functions, too. Those might work better, because even a tireless biting machine can only bite so many people per day.

When might zombie transmission be frequency dependent? FD transmission would be appropriate if larger populations covered larger areas, so that host density was constant. I suppose that could happen if humans were dispersing as much as possible and running away from zombie-packed areas. What do you think?

The Last of Us

I thought that we’d do a quick, just-for-fun post today.  ABOUT ZOMBIES.  Now, I like me some zombie movies.  Sometimes their plots even have cool disease ecology components, like competition between the “zombie virus” and other host pathogens.  On the other hand, there are many, many biological inaccuracies involved in the popular zombie idea, as Neil deGrasse Tyson explains.  

But if you’ve been following this blog or popular science for any length of time, you know that in a way, Neil deGrasse Tyson is wrong in saying that if zombies exist, they only exist on other planets.  There are “zombies” on Earth: parasite zombies!  That is, some parasites can dramatically alter their host’s behavior, so that the host is effectively just a vehicle for the parasite.  The point of this manipulation is to get the host to behave in such a way as to increase the parasite’s probability of transmission to the next host.  Usually, this involves the host getting eaten by the next host, like when infected ants hang out at the top of blades of grass, where they are likely to be eaten by cows or sheep.  But that’s not always the case.  For instance, with rabies – the pathogen most similar to the classic idea of a zombie virus –  the virus makes (some) animals behave aggressively, and this increases the probability that the virus will be transmitted to new hosts via bites.  

Now, thankfully, there aren’t any parasites that re-animate dead corpses…yet.  But there are parasites that use corpses as points of transmission.  For instance, Cordyceps fungus makes ants leave their normal routines to go bite onto leaves above major areas of ant traffic.  Then the fungus sprouts a fruiting body out of the ant’s corpse and rains spores of death down on the ant’s extended family.  

Don’t you think that a Coryceps fungus apocalypse would be a cool video game plot?  Well, actually, it already is a cool video game plot!  The Last of Us has been out for a while on PS3, and a remastered version was just released on PS4.  From Wiki:

“In 2013, Joel (Troy Baker) is a single father living near Austin, Texas with his twelve-year-old daughter Sarah (Hana Hayes). One night, an outbreak of a mutant Cordyceps fungus ravages the United States, which transforms its human hosts into cannibalistic monsters…”

Now, real Cordyceps doesn’t turn insects into cannibals, but I’m willing to overlook this error in biology because – WAIT FOR IT – they’re going to make The Last of Us into a movie, too!  That’s right.  Cordyceps is coming to the big screen.  Awwyisss.