Paper money is a medium of exchange, and also serves as a carrier for germs, traces of cocaine, etc.
I used this medium to spread three different text viruses in the form of rubber-stamped messages. Some anecdotes from this experiment . E. B. of San Francisco returned the dollar and received seven lottery tickets. I sent her the four new tickets and didn't hear from her again. This one received several replies , as it didn't require return of the dollar.