You think your job is tough? Some scientists examined sewage from Pittsburgh, Barcelona and Addis Ababa in a hunt for unknown viruses.
They found scads. How many? At least 43,381.
To put that number into perspective, consider that up to now scientists have charted only about 3,000 viruses. And among the known viruses found in the sewage samples, only 17 were bugs that cause human disease — things like the common cold virus, diarrhea-causing Norwalk virus and human papilloma virus, or HPV, which causes cervical cancer and genital warts.
That means there are a whole lot of mystery microbes out there. These findings appear in the online journal mBIO, published by the American Society of Microbiology.
"Our knowledge of the viral universe is limited to a tiny fraction of the viruses that exist," the study's authors write. This shouldn't be surprising, they add.
After all, Earth is home to around 2 million known species of all sorts, and each species harbors many types of viruses adapted to live on or within its members. Many of these viruses are capable of colonizing other species – or are only a mutation or three away from doing so.
Think H5N1 bird flu, which prefers migrating waterfowl but has "learned" to infect humans when the conditions are right. Or Ebola virus, a monkey microbe that jumps into humans when given the opportunity, with horrific, hemorrhagic effects.
But the presence of such a vast undiscovered panoply of viruses does not mean that there are tens of thousands of unknown diseases waiting to be discovered.
For one thing, the reason human viruses are discovered is that they cause disease. That's a strong signal for scientists to go looking for them, and thus to find them. A virus that can infect humans but doesn't cause disease is much more likely to remain anonymous.
Also comforting is that 80 percent of the unknown viruses lurking in sewage are not interested in humans – or any other eukaryotes, or organisms made up of complicated cells. Four out of five mystery viruses are bacteriophages, which means they infect only bacteria. So unless you're a bacterium, you don't have to worry about them.
And even among the viruses lurking in sewage that infect multi-celled organisms, more than 90 percent infect only plants. That's because humans eat plants and plant viruses dominate the types found in human excrement.
Still, there are undoubtedly important lessons for human health to be learned from the rich microbial diversity in sewage.
For one thing, infectious disease specialists are well aware these days that "new" human diseases can emerge, caused by previously unknown viruses. You may have heard of the human immunodeficiency virus, or HIV, which was unknown until 30 years ago.
And then there's SARS, a particularly nasty microbe of the coronavirus family that burst on the scene in 2003, causing 8,000 cases and 750 deaths. (The current movie Contagion is based on a SARS-like scenario, only worse.)
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.