Most world-historic events - great military battles, political revolutions-are self-consciously historic to the participants living through them. They act knowing that their decisions will be chronicled and dissected for decades or centuries to come. But epidemics create a kind of history from below: they can be world-changing, but the participants are almost inevitably ordinary folk, following their established routines, not thinking for a second about how their actions will be recorded for prosperity. And of course, if they do recognize that they are living through a historical crisis, it's often too late- because, like it or not, the primary way that ordinary people create this distinct genre of history is by dying.

Steven Johnson