Back in the 90's I lived in a part of Washington DC that got water treatment from the Army Corps of Engineers and three or four times I can recall getting the boil-for-five notice. The culprit was always
Cryptosporidium (which is highly resistant to chlorine). The warning usually came after periods of heavy rain that had caused excessive animal waste runoff into the Potomac, though at least once it was due to equipment failure. I always thought that was some pretty 3rd world bullshit, along with the power going out every time it rained.