Sewage is home to thousands of novel, undiscovered viruses, some of which could relate to human health, according to a new study.

The study, published in the journal mBio, was conducted by biologists at the University of Pittsburgh. They looked at untreated waste water, because they knew it would contain a high diversity of viruses.

Viruses are everywhere, but so far only about 3,000 from 84 viral families have had their genetic codes sequenced, write the researchers led by Paul Cantalupo of the University of Pittsburgh.

Researchers, looking for the genetic signatures of viruses present in raw sewage from North America, Europe and Africa, detected signatures from 234 known viruses that represent 26 families, or types, of viruses, making raw sewage home to the most diverse array of viruses ever found.

These unknown viruses could have an impact on human health and environment, said Michael Imperiale of the University of Michigan, who edited the study for mBio, which is published by the American Society for Microbiology.

While some human viruses found in the sewage may be opportunists, lying in wait for their host's immune system to break down, others may be harmless or even helpful. There's a theory out there that we may be infected with viruses that don't cause any disease and may have beneficial effects, Imperiale said.

Understanding more about viruses found in the environment even if they don't cause human diseases can be a very good thing in the surveillance of new public health threats.