Question: How does unclean water affect humans?

Human viruses in water


Viruses are a major cause of human waterborne and water-related diseases. Waterborne diseases are caused by water that is contaminated by human and animal urine and feces that contain pathogenic microorganisms. A subject can get infected through contact with or consumption of the contaminated water.[1] Viruses affect all living organisms from single cellular plants, bacteria and animal to the highest forms of plants and animals including human beings. Viruses can interact with their host through several methods and mechanisms; some viruses can be host specific (HIV) and some can be less host specific (influenza) viruses.[2]  Different viruses can have different routes of transmission; for example, HIV is directly transferred by contaminated body fluids from an infected host into the tissue or bloodstream of a new host while influenza is airborne and transmitted through inhalation of contaminated air containing viral particles by a new host. Enteric viruses primarily infect the intestinal tract through ingestion of food and water contaminated with viruses of fecal origin. Some viruses can be transmitted through all three routes of transmission.
Water virology started about half a century ago when scientists attempted to detect the polio virus in water samples.[3]  Since then, other pathogenic viruses that are responsible for gastroenteritis, hepatitis, and many other virus strains have replaced enteroviruses as the main aim for detection in the water environment.[3]
History
Major outbreaks
Water virology was born after a large hepatitis outbreak transmitted through water was confirmed in New Delhi between December 1955 and January 1956.[4]
Viruses can cause massive human mortality. The smallpox virus killed an estimated 10 to 15 million human beings per year until 1967.[3]  Smallpox was finally eliminated in 1977 by extinction of the virus through vaccination, and the impact of viruses such as influenza, poliomyelitis and measles are mainly controlled by vaccination.[4]
Despite advances in vaccination and prevention of viral diseases, it is estimated that in the 1980s a child died approximately every six seconds from diarrhea confirmed by WHO.[5] Many cases of hepatitis A and/or E, both of which are enteric viruses, are typically transmitted by food and water. Extreme examples include the outbreak of 300,000 cases of hepatitis A and 25,000 cases of gastroenteritis in 1988 in Shanghai caused by shellfish harvested from a sewage polluted estuary.[6]  In 1991, an outbreak of 79,000 cases of hepatitis E in Kanpur was ascribed to drinking polluted water.[3]
A more recent outbreak of Hepatitis E in South Sudan killed 88 people. Medecins Sans Frontieres (MSF) said it had treated almost 4,000 patients since the outbreak was identified in South Sudan in July 2012. In this outbreak, Hepatitis E, which causes liver infections, and was thought to be spread by drinking water contaminated with feces.[7]
Viruses present in water
Sewage contaminated water contains many viruses, over one hundred species are reported and can lead to diseases that affect human beings. For example, hepatitis, gastroenteritis, meningitis, fever, rash, and conjunctivitis can all be spread through contaminated water. More viruses are being discovered in water because of new detection and characterization methods, although only some of these viruses are human pathogens.[4]

Virus survival in water
Viruses need a suitable environment to survive in. There are many characteristics that control the survival of viruses in water such as temperature, light, pH, salinity, organic matter, suspended solids or sediments, and air–water interfaces.
Temperature
Temperature has the highest effect on virus’s survival in water since lower temperatures are the key to longer virus survival. The rate of protein, nucleic acid denaturation and chemical reactions that destroy the viral capsid are increased at higher temperatures, thus viruses will survive best at low temperatures. Hepatitis A, adenoviruses and parvoviruses have the highest survival rate in low temperatures amongst enteric viruses.[3]
Light
Ultraviolet light (UV) is the light in sunlight and can inactivate viruses by causing cross-linking of the nucleotides in the viral genome. Many viruses in water are exterminated in the presence of sunlight. The combination of higher temperatures and more UV in the summer time corresponds to shorter viral survival in summer compared to winter. Double stranded DNA viruses like adenoviruses are more resistant to UV light inactivation than enteroviruses because they can use their host cell to repair the damage caused by the UV light.[3]
Visible light can also affect virus survival by a process called photodynamic inactivation but the length and intensity of the light exposure can change the inactivation rate.[3]
pH
The pH of most natural water is between 5-9. Enteric viruses are stable in these conditions. On the other hand, many enteric viruses are more stable at pH 3-5 than at pH 9 and 12. Enteroviruses can survive at pH 11-11.5 and 1-2, but for only short periods. Adenoviruses and rotaviruses are delicate to a pH of 10 or greater and leads to inactivation.[3]
Virus removal from water
Water that is intended for drinking should go through some treatment to reduce pathogenic viral and bacterial concentrations. As the density of the human population has increased the incidence of sewage contamination of water has increased as well, thus the risk to humans from pathogenic viruses will increase if precautions are not taken.[3]
Scientific studies suggest that the most common viruses found are caliciviruses, astroviruses and enteric viruses. Laboratories are still looking for improved methods to detect these pathogenic viruses. Reducing the amount of viruses in drinking water is accomplished by various treatments that are typically part of drinking water treatment systems in developed countries.[3][8]
Water purification of surface water (water from lakes, rivers, or reservoirs) typically utilizes four treatment stages: coagulation and flocculation, sedimentation, filtration, and disinfection. The first three stages remove mainly dirt and larger particles, although filtration does reduce the number of viruses and bacteria in the water the number of pathogens present after filtration is still considered too high for drinking water. Purification of water from underground aquifers, called ground water, may skip some of these steps as ground water tends to have fewer contaminants than surface water. The last step, disinfection, is primarily responsible for the reduction of pathogenic viruses to safe levels in all drinking water sources. The most common disinfectants used are chlorine and chloramine. Ozone and UV light can also be used to treat large volumes of water to remove pathogens.[8]
Prevention of water viruses
The quality of drinking water is ensured through a framework of water safety plans that ensures the safe disposal of human waste so that drinking water supplies are not contaminated. Improving the water supply, sanitation, hygiene and management of our water resources could prevent ten percent of total global disease.[9]
Half of the hospital beds occupied in the world are related to the lack of safe drinking water. Unsafe water leads to the 88% of the global cases of diarrhea and  90% of the deaths of diarreaheal diseases in children under five years old. Most of these deaths occur in developing countries due to poverty and the high cost of safe water.[9]
Approximately 1.1billion people do not have access to improved water and 2.4billion people do not have access to sanitation facilities. This situation leads to 2million preventable deaths each year.[10]
See also
Waterborne diseases