Just because you drank it and didn't feel a thing doesn't mean it's safe. Pakistani water in many areas is infested with arsenic.
You also have an active immune system but not everyone is same level of competence and will get ill from drinking the same water.
They analyse the waters germ content and Pakistan is so dirty and unclean and full of bad germs. Most population has become immune but weather changes and they get affected (germs numbers increase).
Pakistan has no concept of cleanliness on government level. Look at the shithole of the world Karachi.
This report is 100% accurate.
Most Pakistanis are drinking arsenic water which leads to cancer, brain damage, lung, kidneys, liver cirrhosis, i mean it's poison but at low quantities so people don't even know.
BBC did a report couple years back.