A worker crouches in a San Francisco sewer: Scientists studying sewage have discovered at least 43,381 viruses in the filth.

Liz Mangelsdorf/San Francisco Chronicle/Corbis

t's a dirty job, but, in the name of science, someone has to do it. Sifting through raw sewage may sound disgusting, but it's actually a "goldmine" for scientists charged with discovering new viruses, as evidenced by a new study [PDF] published by the American Society for Microbiology. Here's what you should know:

Scientists really studied sewage?
Yep. Only about 3,000 known viruses had actually been documented by scientists — just "the tip of the iceberg," says the study's editor, Michael Imperiale of the University of Michigan. To identify more, researchers studied sewage from Pittsburgh, Barcelona, and Addis Ababa, Ethiopa.

What did they find?
An incredible number of viruses: At least 43,381 so far. Don't freak out though. There were plenty of known pathogens in the sewage — including viruses that can cause cervical cancer and the stomach flu. And only a small percentage of the sewage viruses were bugs that can cause human disease. Indeed, 80 percent of the newly discovered unknown viruses are only interested in bacteria, and of the remaining 20 percent that attack multi-celled organisms, 9 in 10 infect only plants. "That's because humans eat plants and plant viruses dominate the types found in human excrement," says Richard Knox of NPR.

In that case, what's the value of these discoveries?
"New" human diseases can be caused by previously unknown viruses. Witness the HIV (human immunodeficiency virus) epidemic, or the SARS phenomenon. "Knowing more about what viruses lurk in the environment, even if they don't yet cause human diseases, can be a very good thing in the surveillance of new public health threats," says Knox.

What now?
The big takeaway, says Knox of NPR, is that we're only aware of "a tiny fraction" of the viruses that actually exist in the world. And it turns out there's "a virus-rich universe, ripe for exploration, waiting just at the end of the plumbing."

Sources: U.S. News, NPR, Scientific American