Until the 1970s, many people in Bangladesh got their water from either shallow wells dug by hand or from nearby ponds.
Since such waters are highly vulnerable to organisms that make them dangerous for drinking, death from intestinal diseases was common in Bangladesh.
Tube well systems were decided on as a solution. Tubes were inserted deep into the ground, where there was clean water to be had, thus protecting against waterborne disease. But, over the years, a naturally occurring killer has been silently poisoning many of those wells.
Arsenic, the villain of many a murder mystery, is the villain in this story, too. Its presence wasn't suspected when the wells were built. Some say it was oxidized out of underground soil, as oxygen replaced drawn water.
Others say the water contained a substance that collected arsenic already in the water, and that bacteria have been breaking up those collections, releasing the arsenic that way.
Either way, the devastating effects of long-term arsenic exposure have appeared. The range of possible effects is wide: including skin disease, liver failure, kidney failure, even cancers.
Not all of the tube wells in Bangladesh are hooked up to deadly water, but millions of people have already drunk deeply from those that are. Many thousands of people are reportedly displaying symptoms.
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