An epidemic terror of the end of the world has several times spread over nations.
This section of the book deals with the idea that the world is about to end. It is a perfect dovetail to the modern thriller genre. All of the mechanics of this genre are represented in the great terrors of the past. There are always those who whip up the frenzied masses, either out of ignorance or (more commonly) personal power and profit. Mackay describes the Christian terror that swept through Europe in 1000AD and then goes on to describe similar events surrounding plagues and war, environmental catastrophes (like earthquakes). Quite often, a prophecy is tied to some much earlier prediction of destruction. Modern soothsayers will point to the predictions of Nostradamus or even more latter-day mystics that predict catastrophic geological shifts that will turn Denver into a seaport in a fortnight.
The fundamental mechanism here is that people not only wish to believe these prophesies but that they will encourage the belief by taking part in the lies themselves. We see it when people claim to be abducted by UFOs when they are clearly mad. These are the same folks who would claim to be transformed by witches in the past.