The germs that call New York City’s subways, parks and waterways home are often a reflection of the people who live there and the events that affect daily life, a new study shows.

“You can see a molecular echo of what’s left behind,” said Christopher Mason, the study’s lead author and a geneticist from Weill Cornell Medical College in New York City.

He and his colleagues used nylon swabs to collect DNA from surfaces in New York City’s subways, subway stations, parks and one waterway. Altogether, they analyzed over 10 billion DNA fragments from their swabs.

The fragments of human DNA found on surfaces in the subway reflected the local population.

“The small traces of human DNA left behind on surfaces serve as a mirror or echo of people who move through that station,” Mason said.

In one station that was flooded during Hurricane Sandy in 2012, they found Mother Nature’s mark: germs linked to marine life and Antarctic environments.

While some may think ignorance is bliss when it comes to knowing what’s living on subway railings and turnstiles, the researchers write in the journal Cell Systems that mapping a major city’s germ profile can be helpful in the future.

The findings, Mason said, "establish the first city baseline of microbial life under our fingertips."

“Now that we have this baseline, you can detect strong changes that may determine if there is anything at all threatening,” such as the spread of a disease or bioterrorism, he said.

For example, the researchers now know there are already traces of DNA that match anthrax and the plague on the subway. Future researchers don’t need to worry if they find the same low levels of those germs in any future investigation into bioterrorism.

“In the case of the plague, we see fragments associated with the plague but not strong evidence of the plague itself,” he said, adding that the same is true for anthrax.

Also, the researchers found, about half of all the germs they analyzed for the study had never been seen before. Mason said that may be because they can’t be grown for analysis.

Despite the unknown germs and the possible connections to anthrax and the plague, the researchers say people shouldn’t be afraid to ride the rails or generally touch surfaces around the city.

Even for people with compromised immune systems, such as those receiving intense cancer treatments, Mason said it’s just another reminder to practice good hygiene.

“The majority really of everything you touch represents a very healthy ecosystem that mirrors what’s on your own skin,” he said.