Both faeces and urine can contain infectious microorganisms such as bacteria, viruses and parasites. For the most part, they infect the gastrointestinal system and can be excreted in high amounts when they are shed with the faeces; for example, a billion viruses per gram of faeces. Because some types of organisms survive for a long time or even multiply in the environment, diseases can spread via contaminated water, food or hands long after contact has occurred. In some cases, the diagnosis of an illness can be traced to a specific microorganism. However, in many cases, the source of infection remainsunknown, or a person might even be infected with several types of microorganisms at the same time.
Where sanitation is poor, bacterial infections such as typhoid fever, cholera and shigellosis are common. Cholera can be life-threatening if the infected person is not able to receive health care. Viruses are assumed to be responsible for many undiagnosed cases and rotaviruses are by far the largest cause of diarrhoea in children. This contributes significantly to child mortality in developing countries.
Enteric parasitic infection with helminths and protozoa are of greater concern in developing countries than in industrialised countries. On a worldwide basis, Ascaris lumbricoides is the most common helminth infection, with more than 25% of all humans being infected. Ascaris infections, even in many cases that are not life-threatening, disable nutrient uptake and hamper child development. Protozoa as amoeba, cryptosporidia and giardia are responsible for a majority of enteric infections, causing both illness and death.