Apparently that particular bacteria is basically everywhere on the environment and amounts of it around is pretty harmless. Datacenter just offered a nice and warm environment for it to prosper and then dumped the shitload of bacteria into water treatment system and the treatment plants can't manage that much of it properly.
Also, while it could be deadly, it's more likely that you'll have couple of miserable days on the porcelain throne. But almost any underlying condition (being old, having any kind of gut issues, having flu...) can tip the scale and instead of literally shitty ilness you'll end up in a box.
While Meta is of course guilty here on multiple things one might argue that local government is equally responsible since they allowed Meta to connect their sewer pipe in the first place without proper precautions. But maybe Zuck just had to have a new limousine or whatever so responsibility part was skipped.