Remember when you were young and playing with money, when your mom would tell you to stop it because it was dirty? It turns out she was right all along. You knew it was dirty, but do you know how dirty a dollar bill is?

A dollar bill can contain at least 3,000 different microbes, according to new research by New York University's Center for Genomics and Systems Biology.

The small study was the first step in a large project from NYU to categorize all of the microbes in New York City. Study authors hypothesized that paper money might be a common platform for human-bacteria interaction.

Researchers collected 20 $1 bills in both February and July 2013 (because certain microbes may be more prevalent depending on the season) for the first part of the study. Swab samples were taken on the front and back of the money to see what types of bacteria existed.

They discovered thousands of microbes from the mouth (probably from people licking their fingers while counting), skin (like propionibacterium acnes, which can cause zits) and even some unidentifiable ones that weren't in their databases. But the findings weren't surprising: These types of microbes are found everywhere—not just money, says lead investigator Jane Carlton, Ph.D., a professor of biology at NYU. The team also found more microbes linked to pneumonia from the bills sampled in the winter, which may be more prevalent due to cold and flu season.

For the second phase, Carlton and her team tested 40 $1 bills from September and October 2013 to see if any of the microbes found were viable and could potentially spread infection. In other words, they were finding out how worried we should be of these bacterias found on money.

Get this: Some of the bacteria identified in the first stage of the study—like the skin-related ones—could possibly spread contagious diseases, but further research is needed to determine if you should be concerned.

Carlton insists that you needn't worry much about the currency in your wallet. It just opens up discussions for health departments to analyze money as another mode of transferring diseases. But still, Carlton suggests washing your hands after handling money, the same way you would after using the restroom. Or another option would be to use hand sanitizer. Because seriously, you have no idea where that money has been.

Image via Nikola Solic/REUTERS