We live in a dirty world. Wherever we go, we are among microbes. Bacteria, fungi and viruses live on our phones, bus seats, door handles and park benches. We pass these tiny organisms to eachother when we share a handshake or a seat on the plane.

Now, researchers are finding we also share our microbes through our money. From tip jars to vending machines, each dollar, passed person to person, samples a bit of the environment it comes fromand passes those bits to the next person, the next place it goes.

The list of things found on our dollars includes DNA from our pets, traces of drugs, and bacteria and viruses that cause disease.

The findings demonstrate how money can silently record human activities, leaving behind what are called "molecular echoes."

In April, a study identified more than 100 different strains of bacteria on dollar bills circulating in New York City. Some of the most common bugs on our bills included Propionibacteriumacnes, a bacterium known to cause acne, and Streptococcus oralis, a common bacterium found in our mouths.

The research team, led by biologist Jane Carlton at New York University, also discovered traces of DNA from domestic animals and from specific bacteria that are associated only with certainfoods.

A similar study recovered traces of DNA on the keypads of automated teller machines, or ATMs, reflecting the foods people ate in different neighborhoods. People in central Harlem ate more domesticchicken than those in Flushing and Chinatown, who ate more species of bony fish and mollusks. The foods people ate transferred from fingers to touch screens, where scientists could recover a bit oftheir most recent meals.

Identifying foods that people eat or the drugs people use based on interactions with money might not seem all that useful, but scientists are also using such data to understand patterns ofdisease.

Money laundering
Disease transmission linked to money is rare, and no major disease outbreaks have started from our ATMs. Although it doesn't seem common for diseases to transmit through money, there are ways wecould make our money cleaner.

Researchers are working on ways to clean money between transactions. Putting older bills through a machine that exposes them to carbon dioxide at a specific temperature and pressure can stripdollar bills of oils and dirt left behind by human fingers, while the heat kills microbes that would otherwise linger.

Where's George?
Even if our money is not directly responsible for spreading disease, we can still use the dollar's travel history to track how we spread disease in other ways. WheresGeorge.com, a website created in1998, lets users track dollar bills by recording their serial numbers. In the almost 20 years since the site's creation, WheresGeorge has tracked the geographic locations of bills totaling more thana billion dollars.

Now, physicists at the Max Planck Institute and University of California at Santa Barbara are using data from WheresGeorge to track epidemics. Information on human movement and contact rates fromWheresGeorge was even used to predict the spread of the 2009 swine flu.

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There area lot of gross microbes on a dollar bill(https://www.washingtonpost.com/national/health-science/there-are-a-lot-of-gross-microbes-on-a-dollar-bill/2017/07/28/ba98566e-5b4e-11e7-9fc6-c7ef4bc58d13_story.html?utm_term=.f6be669417cd)