WEDNESDAY, Oct. 5 (HealthDay News) -- Raw sewage contains thousands of undiscovered viruses, some of which could affect human health, a new study suggests.

Viruses are everywhere, but only about 3,000 distinct viruses have been identified worldwide.

In this study, researchers looked for the genetic signatures of viruses present in raw sewage in North America, Europe and Africa. They detected the signatures of 234 known viruses representing 26 different virus families, which makes raw sewage home to the most diverse collection of viruses ever documented.

Known viruses detected in raw sewage included human pathogens such as human papillomavirus, which can cause genital warts and cervical cancer, and norovirus, which causes stomach flu. There were also viruses associated with rodents and cockroaches, viruses from plants and viruses that prey on bacteria.

But most of the genetic signatures belonged to unknown viruses, many of which play unknown roles in human health and environmental processes, according to study editor Michael Imperiale of the University of Michigan.

While some of the unknown viruses from humans may be harmful, others may be benign or even helpful, he said.

The study was published Oct. 4 in the online journal mBio.

The researchers plan to look for viruses in other environments around the world.

"I think this is going to be the tip of the iceberg of how many viruses are out there," Imperiale said in a journal news release. "I think the ocean is going to top raw sewage by orders of magnitude," although viruses won't be found in such densities as they are in sewage.

More information

The University of California Museum of Paleontology has more about viruses.