Pathogens can enter our bodies through four routes.
Since our interest here is drinking water, the route of exposure we will consider is ingestion. Common symptoms of gastrointestinal diseases are vomiting and diarrhea. Prolonged and severe diarrhea is a major cause of death in many parts of the world, and contaminated, untreated drinking water is a principal cause of those afflictions. Invasion by pathogenic microorganisms of a bodily part in which conditions are favorable for growth, production of toxins, and subsequent injury to tissue.