http://www.nytimes.com/2009/12/15/health/15real.html seems to indicate for a surface, to serve as a medium, the following properties are relevant
The article however does not elaborate on the properties a surface must possess to serve as a medium - albeit i would figure it may vary from one virus/bacterium to another.
The same article also states
... when objects in a hotel room — light switches, telephones — were contaminated with a cold virus, 60 percent of healthy volunteers picked up the virus when they touched one of the objects an hour later. Eighteen hours later, the transmission rate was cut in half.
When currency changes hands we rarely ever know whether it may , within a reasonable timespan, have passed through an agent/carrier.
Sir Dumpty loves to sneak off with a currency note to get me to chase him. He is protected; another member of his species may not be so protected. For that matter a person infected with, say, the common cold virus may have passed a note/coin/card to the vendor at the store within a minute of sneezing/coughing into his/her hand. What if the same person , instead, suffered from TB?
So ... can currency serve as a medium for transmission of pathogens?