The Slaves of House Hezzrat
In Athas, water is as precious, and sometimes more so, than gold. Its a commodity that is bought and sold with blood as often as ceramic, with the value of a muddy puddle sometimes counted in the bodies willing to defend it.
Because of that, the average Athasian has turned into something of a super-being. Centuries of being forced to consume the most terrible of waters has granted them a great endurance for the sicknesses that crop up in waters. What used to cause great dehydration through loss of liquids and electrolytes through vomiting and diarrhea, now, only results in painful cramping and dizziness. while the sickness is potent, and will still create problems for a traveler desperate for water, the sickness itself will not kill the consumer of fouled water.