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?