one: both eventually plateau, but the VCR curve grows exponentially
at first. Simple models of contagion will usually predict this kind of
growth, because each new adoption creates even more adoptions,
whereas models of independent happenings will not. It doesn’t mean
that exponential growth is always a sign that something is contagious
- there might be other reasons why people increasingly adopt a
technology – but it does show how different infection processes can
affect the shape of an outbreak.
If we think about the dynamics of an outbreak, we can also identify
shapes that would be very unlikely in reality. Imagine a disease
epidemic that increases exponentially until all of the population is
affected. What would be required to generate this shape?
In large epidemics, transmission generally slows down because
there aren’t many susceptible people left to infect. For the epidemic to
keep increasing faster and faster, infectious people would have to
actively start seeking out the remaining susceptibles in the later
stages of the epidemic. It’s the equivalent of you catching a cold,
finding all your friends who hadn’t got it yet and deliberately coughing
on them until they got infected. The most familiar scenario that would
create this outbreak shape is therefore a fictional one: a group of
zombies hunting down the last few surviving humans.