Worldwide, water pollution is the single greatest cause of human disease. Waterborne diseases such as cholera, typhoid fever, and dysentery kill 25 million persons annually. The main source of these pathogens is untreated human and animal wastes that are dumped into waterways. It is estimated that 2.5 billion people lack sanitation and over 1 billion lack clean drinking water. The quality of water can be assessed by measuring the level of coliform bacteria. These are the bacteria found naturally in the human colon (large intestine), and make up almost 50% of the matter in feces.