We no longer let people pet ours for this reason...Even when we sell them, you can pet what you bought...Baby ducks are the biggest draw for touching, in our experience, we now keep them in enclosed cages when we're out selling them...
Ideally any bird that has been off your property , whether it be at a show or sale , should be quarantined. I don't allow anyone to walk in my pens and after seeing unwell birds at a local show , I don't show either.