Scientists are still not quite sure how it happens. Some kind of microorganism, most likely; boiling makes water safe again, and rainwater (as long as it doesn’t touch the ground) doesn’t cause any problems.

But any groundwater? Ponds, rivers, oceans? Even puddles after a thunderstorm? Deadly. Not right away, no, or else the death toll wouldn’t have been so high. But the slightest skin contact causes a painful death a couple days later. Only in humans, though; no one has been able to explain why.

Now that most of the population is gone, society has broken down, and the streets are no longer properly maintained, a spring rain can turn the city into a game of Floor Is Lava. It’s almost funny, seeing the few people left trying to go about their lives, hopping and skipping around puddles. After centuries of laziness, humanity is having to trust their feet again. Just watch your landing: the pavement is wet, you don’t want to slip and fall on your ass.