Money changes hands and attracts lots of bacteria. A joke is:
 
“They say money has germs, but even a germ couldn’t live on the money I make.”
 
The joke has been cited in print since at least 1935.
 
         
Google Books
Doctor Bosko’s Invention;
A novelty in five skits
By James C. Parker
Memphis, TN: National drama Co.
1935
Pg. 54:
THUN.—Dere ain’t no germs on my money.
INT.—What makes you think there are no germs on your money?
THUN.—Because, Mr. Joe, even a germ couldn’t live on the money I make.
 
Google Books
3650 Jokes, Puns, and Riddles
By Anne Kostick, Charles Foxgrover and Michael J. Pellowski
New York, NY: Black Dog & Leventhal Publishers, Inc.
1998
Pg. 242:
Gina: They say you should wash your hands after handling money, because the bills are covered with germs.
Sophia: I’m not in any danger—even a germ couldn’t live on the money I make!
     
Google Books
Extremely Good Clean Jokes for Kids
By Bob Phillips
Eugene, OR: Harvest House Publishers
2001
Pg. 83:
Jennifer: They say you should wash your hands after handling money, because the bills are covered with germs.
Julie: I’m not in any danger—-even a germ couldn’t live on the money I make!
     
Twitter
Bob Fielder‏
@bfielderobx
You know money has lots of germs and can make you sick…..naw…germs couldnt live on the money I make! HEE HAW. http://twitpic.com/4o6nhv
6:19 PM - 22 Apr 2011
 
Twitter
The Doctor‏
@TheDrDon
I don’t worry about germs when handling money. Even germs couldn’t live on the money I make…
6:00 PM - 9 Dec 2011
 
Twitter
Fkn Ricky‏
@BigRich027
They say that most paper money is covered in germs. But I’m not worried. Not even a germ could live on the money I make!! #broke
4:54 PM - 9 Feb 2016
 
Reddit—Jokes
They say money is covered in germs
submitted July 1, 2017 by saxophonefartmaster
But I don’t think even germs could live on the money I make.