they contain all kinds of stuff

The average lifespan of a US dollar bill is 18–22 months, roughly 45% of all bills printed each year are dollar bills.

I would assume most bills have some trace of something on them, whether it be viruses or cocaine or just dirt.

Ha! I just went on a tour of the NY Federal Reserve bank a couple of weeks ago. The nice tour guide there said that it’s about a year and a half to two years of service for a bill, then they’re pulled out of circulation and taken to the nearest Federal Reserve Bank to be shredded and burnt. As for viruses, that depends on how long they last in the open air between handlings, I’d reckon.