I'm no doctor, but I tend to believe that you can NOT reinfect yourself with your own viruses or bacteria that have been deposited on a harmonica. I believe that once your immune system has defeated a germ, that germ has no chance of making a successful comeback because your immune system knows it and is ready for it. That's not to say that you might not feel some effects of the "battle" that will take place when the re-infection is attempted, but its a battle that particular germ will not win unless your immune system is compromised in which case all bets are off.