A few years ago Christopher Mason, a geneticist at Weill Cornell Medical College in New York City, dropped his eight-month-old daughter off at daycare and watched as she put a plastic toy giraffe in her mouth. Then he watched that giraffe go into another kid's mouth. And then another. “It got me thinking about what microbes were being transferred,” Mason says. Looking around New York, he realized: Critters live on every surface people touch, all the time. Especially, he realized, on the subway.

By 2013, Mason's vision of a city teeming with shared germs had become a project. He recruited grad students and other researchers to same the entire New York City subway system. After 18 months of swabbing surfaces of metal handrails, turnstiles, ticket kiosks, and other places that people touch a lot, the team had what they call a PathoMap, the first description of New York's subway-riding microbial denizens. In a city where 5.5 million people ride the underground (and occasionally elevated) rails among 466 stations every day, this is data that both allays and inspires dread.

The newly published map provides a snapshot of a city teeming with microbes—septillions of them (that's a one followed by 24 zeroes). No need to grab your hand sanitizer or stock up on antibacterial soap, though. (In fact, don't; that stuff is terrible.) While some of the microbes were pathogenic—including traces of anthrax and the Bubonic plague, which, believe or not, were harmless—straphangers have nothing to worry about. Many of the species are good for human health or simply associated with foods like cheese and yogurt. “This is what a healthy city looks like,” Mason says. It's also a baseline that could give public health experts and city planners a new way to look at urban areas, tracking microbial ebb and flow in real time.

The researchers found traces of at least 637 known species of bacteria and a smattering of viruses, fungi, and microscopic animals. They also found...well, dunno. As the New York Times noted, about 48 percent of all the samples didn't match any known species. It turns out that kind of uncertainty is pretty common in microbial censuses. In a 2013 analysis of all the DNA in a bunch of high-volume samples of the outdoor and indoor air in San Diego and New York, a research group lead by the famed geneticist Craig Venter failed to identify nearly half the samples. Still, when you think about it, that's kind of weird. "I would've thought that we knew the majority—maybe 80 to 90 percent—of the microbes in a human environment," Mason says. "It means there's a lot left to be discovered."

These kind of microbial maps aren't just for show. Done right, they can reveal a lot about a city's history and future. As Robert Lee Hotz wrote in his long-in-the-works account of the research for the Wall Street Journal, it's "a revealing glimpse into the future of public health." For example, at the South Ferry station, the researchers found 10 species of marine microbes that they didn't detect anywhere else—leftovers, the researchers suspect, of flooding from Superstorm Sandy in 2012. And in busier stations and more densely populated areas of the city, the range of bugs also went up. “More people brings more diversity at the microbial level,” Mason says.

Eventually, maps like Mason’s could even guide the construction of so-called probiotic buildings. Mason’s team found that some of the bacteria endemic in the subway were species that could help clean up hazardous chemicals. Others were vital to the human immune system. The researchers imagine a day when they understand patterns of microbial behavior well enough to create places those helpful germs want to live.

With that kind of potential, it makes sense that more microbial maps are in the works. Last November, a team in Hong Kong plotted the bugsin that city’s subway system—but unlike the PathoMap, which looked at microbes living on surfaces, the Hong Kong group looked at what lived in the air. That’s the difference between what you can touch or what you can breathe in. “It’s all part of this larger, emerging understanding that, wait a minute, we humans aren’t just occupied by a large number of bacteria—we live in a sea of them,” says Gary King, a microbial biologist at Louisiana State University. (King wasn’t part of either subway study.) “They’re everywhere around us, and sometimes they affect our health adversely, and sometimes they have beneficial effects on our health.”

Mason is now planning to make similar maps of 20 of the busiest urban subway systems around the world, part of a project called MetaSUB. Researchers have already started collecting samples in Paris, São Paolo, and Shanghai. The subway says more about cities than just where people travel. “It’s a communal ecosystem that’s never been studied at great depth in terms of something that we touch every day,” Mason says. We travel in it, and it travels within us.

Explore an interactive map of the microbes in New York City’s subway system here.