Speaking of contaminated water, drinking the stuff can lead to a miserable bout of stomach pain and loose bowels, as many of us who've traveled in the developing world can attest. But water-related illnesses do more than just ruin trips. 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 [source: VOA].
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: Encyclopaedia 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. Among other pioneering public health ideas, he came up with the suggestion 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 much safer [source: Lenntech].