In a search for new viruses, researchers are hitting the streets -- or under them, at least.

After examining wastewater samples from three continents, scientists discovered that raw sewage holds the most diverse array of bacteria of any place collected from so far. Until now, virologists, or researchers who study viruses, identified and described approximately 3,000 viruses, but most think current numbers barely skim the surface of all the viruses found on Earth.

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The survey, which required sequencing the DNA of samples taken directly from the untreated wastewater, may act as a starting place to understand unknown viruses and their hosts. Since sewage is representative in nature -- that is, it consists of samples from a wide range of people, it can also clue scientists in on which viruses and bacteria are most common in humans and animals. Who knows, some viruses may even prove helpful to animals or ecosystems.

Scientists gathered samples from Pittsburgh, Pa., Barcelona, Spain and Addis Ababa, Ethiopia to find traces of 17 viruses known to infect humans, such as the human adenovirus, Norwalk virus (responsible for stomach flu), the human papillomavirus and the human polyomavirus, to name a few.

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The samples were also chock full of bacteriophages, or viruses that infect bacteria, and roughly 90 percent of eukaryotic virus results were plant-based. Both findings suggest the viruses stem from a variety of sources, including animal and human feces and urine and plant material from domestic and agricultural settings. Some DNA matched up to viruses commonly found in insects and rodents -- frequent residents of sewer systems.

But the majority of the DNA from samples didn't match known viruses, leading scientists to believe many are yet to be discovered. By examining specific points on the samples' genomes, the team could determine whether a specific virus had similar genes to other known types.

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The study appears in the journal mBio, published by the American Society of Microbiology. The study was edited by Michael Imperiale, a researcher at the University of Michigan.

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