Washing your hands before playing the piano used to be a stringent rule in days gone bye, usually enforced by grannies who knew how important it was.
I frequently have to tune pianos with years of accumulated visible dirt on the keys,which I generally have to clean with surgical spirits before getting started. Consider all the children with illnesses who pass through the music department at schools who would be leaving their germs on the keys.
I would be interested to have a test conducted as to the presence of harmful bacteria on the average piano keyboard. I am sure we will be surprised !