Oh, it’s inside a pool! Yes, the nearly horizontal surfaces are the border tile, the transition tile and the pool bottom tile.
It does make sense. But I’m not sure why one would want 3 different tiles in a pool (the vertical, transition, and bottom).
Yes, and we know how to treat it (not cure, but we know how to make people not die) since, well, before the times depicted on the game. We have also knew how to avoid most of its infections since… hum, I don’t know that one, but Ancient Rome was full of structures made for that, and functional, so some time before them.
Still people didn’t use to be able to afford treatment (that is basically drinking large amounts of clean water with salt and sugar, and resting). And it takes a functional government to avoid it.