Yesterday, Isoroku and I spent several minutes arguing over whether I should pick up a dime that I spotted near a gas station garbage can. (Spoiler alert: I did!)

On the one hand, ten cents is ten cents. On the other hand, ground money is filthy. In fact, here’s currency expert and former IMF chief economist Kenneth Rogoff:

Researchers at New York University analyzed $1 bills and found evidence of thousands of microbes, including a wide range of bacteria, even some antibiotic-resistant ones. Though most people are aware of the hygiene problems associated with handling cash, one can imagine paper currency being an agent of transmission in some future pandemic.

OH MY GOD.

This belief was popular at the end of the 19th century, when leprosy was thought to be highly contagious and patients were quarantined in isolated colonies. Healthy people worried about touching money that had been in contact with leprosy patients, so many governments issued separate currency only for use in leper colonies.

The US operated the Palo Seco Leper Colony in the Panama Canal Zone. Palo Seco coins were backed by US currency deposits and corresponded to US coins in size and weight, except that they had holes. Leprosy affects peripheral nerves, and holed coins made it possible for patients to string them together for ease of handling.

Here are some more pictures of leper colony coins.

Today, we know that leprosy isn’t transmissible by money. Most doctors knew this back then as well. The differentiated currency was mainly an excuse to further segregate leprosy patients and prevent them from escaping from their colonies.

Currency is a poor agent for disease transmission. First of all, human skin is naturally covered in thousands of strains of bacteria (even antibiotic-resistant ones!), many of which play a protective role by competing against pathogens and suppressing fungal growth. A deadly microbe on a dollar bill would have little chance of infection unless people are stuffing currency into their respiratory passages. (Or picking their nose and rubbing their eyes after handling money. Don’t do that.)

Even then, pathogens don’t transfer well on porous surfaces like paper or fabric*. Viruses and bacteria actually survive much longer on hard surfaces like plastic credit cards. Every time you insert your credit card in a card reader, you’re picking up germs from every other credit card that reader has processed. One can imagine credit cards being an agent of transmission in some future pandemic.

*Note: This statement comes from the National Health Service in the UK, but it seemed suspect so I consulted a microbiology professor at USC. He speculates that the experiment was conducted by spraying viruses on different surfaces and measuring the recovery efficiency after different lengths of time. A porous surface like paper would have a low recovery rate because viruses get pulled into the fibers, where they suffer protein degradation until they are effectively dead.