The chemical property of copper, zinc, nickel, and tin also keep a microbe from mutating. Stainless steel does not. You are relatively safe with coins but paper currency could harbor a virus for days. Use a debit card. If you use cash and receive cash as change, it is easy to spray it with an antimicrobial spray, allowing it to dry and use it again. I have an infectious disease doc as a client who would bend over to pick up a penny but has not used cash since medical school.