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It has long been suspected that cash (banknotes and coins) might "[..] serve as a vector of disease, though researchers disagree over how easily diseases are transmitted this way." (source Wikipedia)

There are very few studies which have systematically looked at the extent to which cash might be transmitting diseases.

A 2014 study published in Future Microbiology found that :

[…] the potential for banknotes and coins to carry bacteria and fungi, as well as their potential capacity to spread infectious agents. In addition, banknotes and monetary coinage can act as potential reservoirs for antibiotic-resistant bacteria, such as MRSA. Many food handlers do not give much attention to hygienic practices, and money recovered from food handlers was highly contaminated."

Adding :

The capacity of banknotes, coins and fomites to serve as sources of pathogenic agents represents a major challenge in the 21st century."

See : Paper money and coins as potential vectors of transmissible disease Future Microbiology 9(2):249-61.

Note: A 'fomite' is any physical object which can harbour pathogens and play a role in disease transmission.