According to an  article  appearing in the American Society for Microbiology's journal,  mBio , a group of raw sewage-examining scientists have discovered more than 43,000 viruses in the wastewater of a mere three cities: Pittsburgh, Pennsylvania; Barcelona, Spain; and Addis Ababa, Ethiopia. This number might not sound like much. After all, there must be thousands of known viruses, right? Indeed, there are. Scientists have charted roughly 3000 of them. It's just that the good folks behind these findings have found a minimum of 43,381 bugs lurking in the filth-lined sewers beneath the streets of just three cities. But before you begin panicking and digging through your dusty volume of Nostradamus, it is important to note that even the microbiologists authoring the  mBio  article dismiss their findings as "unsurprising." Virologists have made it plain that our present knowledge of viral diversity is, at best, limited to a very small fraction of the viruses in the world.