As many of us who've traveled in the developing world can testify, drinking water that's contaminated with pathogens can lead to a miserable bout of stomach pain and loose bowels, and a hasty trip to a local medical clinic. But water-related illnesses do more than just ruin vacations. As the World Health Organization reported in 2005, such diseases are the world's leading cause of death, claiming 3.4 million lives annually -- more than war, terrorism and weapons of mass destruction combined. Children in impoverished countries, whose immune systems already are weakened by malnutrition and other stresses, are particularly at risk; about 4,000 of them die each day from drinking filthy water [source: VOA].
But it used to be even worse. For centuries, even in developed countries, mysterious periodic outbreaks of water-borne cholera regularly killed many thousands of people [source: Britannica]. During a cholera outbreak in 1854, British scientist John Snow determined that the disease was caused by microorganisms in sewage that contaminated the water supply. He came up with the idea to apply chlorine to the water to kill the microorganisms, and the illness rate plummeted. Since then, additional chemical and filtration technologies have been developed to make our drinking water safe [source: Lenntech].