It sort of starts out as a minor itch and then you get this red spot. The more you scatch it... the more red spots you get. Before you know it, you have white spots and yellow spots and blue spots and eventually even black spots!
By then it is too late... you have full blown Exonumia. http://en.wikipedia.org/wiki/Exonumia#Forms_of_Exonumia:_Tokens_and_Medals