Prions can lie in wait for years (or decades) before manifesting in symptomatic cases. That's part of what makes them so dangerous--vast swathes of the population can be infected before the first person starts showing symptoms. Most of them also don't have any conclusive non-destructive diagnostic test. They're diagnoses of exclusion and symptom; the only way to know for sure whether or not any particular case is caused by a prion is to do an autopsy after death. Those characteristics together make it the perfect "stealth" plague: invisible until the first cases, and already ubiquitous by then, with no known treatment or even diagnostic instrument.

If we wanted to get really "English-teachery" about it, you might compare that kind of functionality to ideology, and think about the role that ideology plays as a behavior motivator (and historical Prime Mover) under capitalism (that is, under ultraliberalism and moralism). Joyce rejects it as a causal agent of the revolution because she is, as others have noted here, actually a materialist and it is a stand-in for ideology. I have no idea if the devs meant that parallel to be drawn, but it's an interesting reading.