Viruses are ultramicroscopic organisms. They are parasites; they need to infest a host in order to duplicate themselves. Viruses excreted with human and animal feces are called enteric viruses, and more than 100 such organisms have been identified. As many as one million viruses can be found in one gram of excrement. The concentration in raw sewage varies over a wide range; as many as 463,500 infectious particles per liter of raw sewage have been detected. Viruses found in surface waters are introduced from three major sources. Viruses of human origin can be traced to untreated or inadequately treated domestic sewage. Runoffs from agricultural land, feedlots, and forests introduce viruses from domestic and wild animals and birds. Plant viruses, insect viruses, and other forms of life associated with the aquatic environment may also infect the waters.
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