Problems that make water undrinkable include contamination by debris, microbes, toxic chemicals and radiation. Drinking water containing these contaminants can cause diarrhea, disease and death.
Debris such as dirt and parts of decomposing plants and animals affect the appearance of water and make it smell and taste unpleasant but do not always make it unsafe to drink.
Bacteria - especially coliform bacteria that live in the intestines of animals - may indicate the presence of pathogens that cause fecal-borne illnesses such as cholera and typhoid. Viruses in the water cause hepatitis A, viral meningitis and dengue fever. Worms and other parasites may cause giardia and hookworm. The amoeba Acanthamoeba causes keratitis in the eye, which can lead to blindness.
Toxic chemicals in water include naturally occurring and man-made chemicals. Ingesting them may lead to poisoning by lead, mercury, arsenic and benzene. Nitrate can impair the blood's ability to carry oxygen to cells throughout the body, resulting in cyanosis and death, especially in babies.
Radioactive chemicals such as radon and plutonium in water can cause cancers. The children of people who drink the contaminated water may be born with birth defects and health problems.